If you are very adventuresome: I understand that it's possible to walk up the hillside from Black Mountain Road, just south of Oviedo, but I've never tried this.  At the top, you may need to walk a ways around the field to get through the fence.

CARLSBAD – (Game Location – Pacific Rim Elementary School) – 1100 Camino De Las Ondas
Take 5 North to Poinsettia Lane and turn right (east). 
Take a left on Batiquitos Drive. 
Turn right on Camino De Las Ondas. 
Pacific Rim Elementary will be on your left. 
Field is at the north end of the school.

Carmel Del Mar School (CDM) – (map) 12345 Carmel Park Drive
from I-56 westbound: take the Carmel Creek Road exit.
Turn right on Carmel Creek Road.
Turn right on Carmel Grove Road.
Turn right on Carmel Park Drive.
Continue to the bottom of the hill and park near St. Agnes Court.
There's a walkway leading into the park/school on the north side of Carmel Park Drive.

from I-56 eastbound: take the Carmel Creek Road exit.
Turn left on Carmel Creek Road.
Turn right on Carmel Grove Road.
Turn right on Carmel Park Drive.
Continue to the bottom of the hill and park near St. Agnes Court.
There's a walkway leading into the park/school on the north side of Carmel Park Drive.

Carmel Valley Middle School (CVMS) – (map) 3800 Mykonos Lane, San Diego
from I-56 westbound: take the Carmel Creek Road exit.
Turn right on Carmel Creek Road.
Turn left on Mykonos Lane, and go to the end into the school parking lot.

from I-56 eastbound: take the Carmel Creek Road exit.
Turn left on Carmel Creek Road.
Turn left on Mykonos Lane, and go to the end into the school parking lot.

CORONADO HIGH SCHOOL – (map) corner of 6th Street and F Avenue  (aerial photo)
Note: the 800 6th Street address you will see on Yahoo Maps by clicking the link above isn't a real address, but it puts a red star on the map exactly in the location of the field.
5 Freeway South to CA-75 exit towards Coronado. 
Go over the Bay Bridge. 
Continue on 4th Street.  Bear right on Pomona Ave. 
Turn LEFT onto Orange Avenue. 
Turn RIGHT onto 6th Street. 
In 3 blocks you will see the stadium on the left at F Avenue.
Park on 6th street or F Avenue, the game is in the stadium.

DOYLE REC CENTER – (map) 8175 Regents Rd.
I-5 South to La Jolla Village Drive.  Turn left (East) on La Jolla Village Drive.  Turn right on Regents Road.  Doyle Park and Rec Center will be on your left after you cross Nobel Drive.

EARL WARREN MIDDLE SCHOOL – 155 Stevens Ave.
5 North to Lomas Santa Fe Drive, turn left. 
Turn left on Stevens the school is on the corner or Lomas Santa Fe and Stevens Ave.

FALLBROOK HIGH SCHOOL – (map) 2400 S. Stagecoach Lane
Take I-5 North, Exit at Highway 76 East. 
Go east on Highway 76 for about 12.2 miles (you will go through Oceanside & Bonsall)
Turn left onto Mission Road, and go north about 4.1 miles. 
As you pass the signal at Winter Haven you'll be able to see the high school to your right. 
Turn right onto StageCoach Lane.
The field is on your right, just past the school buildings.
(approximate travel time is 59 minutes)

LA COSTA CANYON – (map) 3451 Camino de los Coches, Carlsbad (aerial photo)
I-5 North to Leucadia Blvd. 
Turn right and go for about 3 miles to Camino de los Coches.
The school is on the right about ½ mile down.

LA JOLLA HIGH SCHOOL – (map) 750 Nautilus St., La Jolla  (aerial photo)
Take I-5 South to the LA JOLLA VILLAGE DR exit. 
Turn RIGHT onto LA JOLLA VILLAGE DR (go West), then go 0.7 miles
Turn LEFT onto TORREY PINES RD., go 3.2 miles 
Turn LEFT onto GIRARD AVE., go to the next light
Turn RIGHT onto PEARL ST., and the next turn is almost immediately on your left 
Turn LEFT onto FAY AVE, go 0.5 miles
There appears to be parking on your right off of Fay Ave just before you reach Nautilus St., at the south end of the stadium.

MT. CARMEL HIGH SCHOOL – (map) 9550 Carmel Mountain Road, San Diego
Take I-56 East.  I-56 becomes Carmel Valley Road, and continue to Black Mountain Road and take a sharp right turn onto Black Mountain Road. 
Turn Left on Carmel Mountain Road. The school will be on your left.

PACIFIC RIM ELEMENTARY SCHOOL – (map) 1100 Camino de las Ondas, Carlsbad
I-5 North to Poinsettia Lane.  Turn right onto Poinsettia Lane.  Turn left onto Paseo del Norte.  Turn left onto Camino de las Ondas. 

PATRICK HENRY HIGH SCHOOL – (map) 6702 Wandermere Dr., SD, 92120
I-8 (east or west) to College Avenue (North)
College Avenue until you get to Navajo Road
Turn right on Navajo Road
Turn right onto Park Ridge Blvd.
Turn right onto Wandermere Drive
School is on the right, pull into the first driveway
Walkway to the stadium is at the back of the parking lot to the left

POWAY HIGH SCHOOL – (map) 15500 Espola Road (aerial photo)
Take I-56 East.  I-56 becomes Carmel Valley Road.
Continue for about 4.4 miles from the end of I-56. 
Turn RIGHT onto Camino Ruiz and go for about 1.5 miles. 
Turn LEFT onto I-56 east, and continue and cross over I-15 (I-56 becomes Ted Williams Pkwy).
Turn RIGHT onto Twin Peaks Road and continue for about 2.2 miles. 
Turn LEFT onto Espola Road and continue for about 1.3 miles. 
The school will be on your left. 
Turn into the last driveway. Stadium is on the right hand side (north end) of the school. 
Parking is in the front of the school or around the back. 

RANCHO BERNARDO HIGH SCHOOL – 13010 Paseo Lucido  (aerial photo)
Take I-56 East.  I-56 becomes Carmel Valley Road, and continue for about 4.4 miles from the end of I-56.  Turn right onto Camino Ruiz and go for about 1.5 miles.  Turn left onto I-56, and continue to I-15.
Go North on I-15 until Camino Del Norte. 
Turn Right on Camino Del Norte and left at Paseo Lucido. 
High School and Middle school are on your right hand side. 
The field is in between the two schools behind Auditorium/Performing  Arts building straight ahead when you enter the driveway.

SAN DIEGUITO ACADEMY – 800 Santa Fe Drive, Encinitas
5 North to Santa Fe Drive. 
Turn right, the school will be on your left about 2 blocks up.

SANTANA HIGH SCHOOL – (map) 9915 Magnolia Avenue, Santee
I-5 south to I-805 south.  Merge onto highway 52 and go east.  Exit at Mast Blvd.  Turn left onto Mast Blvd.  Turn left onto North Magnolia Avenue.

SERRA HIGH SCHOOL – 5156 Santo Road
805 South to 52 East. 
Take Santo Road exit, the school will be on your right about a mile down.

WESTVIEW HIGH SCHOOL – (map) 13500 Camino Del Sur, San Diego  (aerial photo)
Take I-56 East.  I-56 becomes Carmel Valley Road.
Continue for about 4.4 miles from the end of I-56. 
Turn RIGHT onto Camino Ruiz and go for about 1.1 miles. 
The school will be on your right.