[PaxMacian]

No problem, just divide all the existing rooms in half making two rooms then squeeze in a few more kids into each.

[NYTexan]

"just divide all the existing rooms in half"

Actually you are on track.
The simple thing to do is go to split sessions.
Devide the class in half, teach intensively for 1/2 day each, then herd them into the cafeteria, gym, outside, whatever for study hall / reading assignments, lunch, and gym.

Bingo: not much more added expense!