I'm 11 years old and in the alto section of our choir,but I can't get past the C past middle C (is that C4?) because my voice is breaking, but I can't really sing in the tenor range, and a year 7 tenor is a bit unconventional in our school anyway.

Howver the year 7 sopranos have all been taught falsetto so they can now sing b above the stave (and they're 13)

I used to be able to access head voice but all I get now is white noise

Does anyone have any falsetto tips?

IMO it could be a little too soon. You should wait until you voice changes more completely. As a beginner Tenor you should sing from about a low C to a high G or higher ("low C" for me is the one 1 line under the stave and "high G" the one immediately over the stave (as played with a piano) Later you will need to reach A easily and then Bb, B and C (two octaves).

With Falsetto you should be able to sing as Alto (what you have been doing so far). But why do you want that?