I don't understand:
the school has told us that we have to be alone when we hour build
If you have a FAA Airman's Certificate, this, subject to a few other caveats, qualifies you to fly N-registered aeroplanes with passengers on board, and for the purposes of FCL.050, that's what your buddy is, a passenger.
Your school is free to impose any restrictions it likes, but I cannot see a problem from an FAA licensing point-of-view.