As PJL says you can simply push the ISO 400 film one or two stops to expose it at EI 800 or 1600. (Which film have you got and how do you use the D-76 at school – diluted one-shot; reused or replenished?) Even Ilford Delta 3200 needs to be pushed to get EI 3200 because it is an ISO 800 or 1000 film (depending on the developer) not an ISO 3200 film (Ilford are careful not to call it an ISO 3200 film). To the best of my knowledge there are no ISO 3200 B&W films – they are around ISO 800 to 1250 and need to be pushed when exposed at EI 3200.
You can push colour film, both negative and reversal. You usually define the number of nominal stops of push when sending it in for processing. This may not correspond exactly with the EI setting on your camera. For example, I would expose ISO 320 reversal film at EI 1000 instead of EI 1250 when specifying ‘Push 2’ and at EI 1600 or 2000 when specifying ‘Push 3’. For reversal film you can fine-tune the number of stops or fractional stops of push: ie Push 1/3, Push 1/2,Push 2/3, Push 1. Push 2, Push 3. For colour neg I find that it’s not really worth doing anything less than Push 2.
400 speed film may be OK developed normally – it depends on the lighting and the maximum aperture of your lens. When I first started doing concert photography the fastest colour film we had available was ISO 160.
Best,
Helen