Forwarding X Through Several Hosts When Configurtion Prevents It

I have computer A at my home, and at school there is computer C, which I want to run X applications and forward to my computer A. Except, Computer C is not accessible from outside the school's network. To get to Computer C, I have to first ssh to Computer B, then ssh to Computer C. Except that Computer B prevents forwarding X from C to B to A. Here is a workaround I found.

First set up a tunnel

ssh -L5091:machine_C:22 username@machine_B

Then ssh by:

ssh -Xp 5091 username@localhost

Test with xclock.