It depends on where you're coming from; post a detail to say roughly where you're coming from (a borough, a neighborhood, something like that - you don't need to post an address).
That's Springfield Gardens High School, I'm familiar with the area, been there a bunch of times. It's served by the Q77 and Q85 buses. I'd estimate that once you get on the bus, it would take between 20 and 30 minutes to get there, maybe a bit more. I can only suggest that you try it to see exactly how long it takes.
As for getting to there ONLY by train, I'm sorry to say that the subway is really not close to there. HOWEVER, the Long Island Railroad's Laurelton station is about two blocks away; I've walked it a few times, you can see the station from the school's field.
Of course, the LIRR will cost a bit more than the subway/bus, but it'll certainly be faster, especially if you live near a LIRR station on a line that connects to the Far Rockaway Branch. Being on the Port Washington Branch (Shea Stadium, Flushing/Main St., Murray Hill, Broadway, Auburndale, Bayside, Douglaston, Little Neck) is not good enough.
All in all, if you don't take the LIRR, you'll have to take a bus/train combination. At least the E and F trains move VERY quickly through Queens.
