"When I flew solo to another airfield as part of my PPL I was taxiing to the hold when I realised I had made a silly error. This threw me and then I became conscious of another bigger aircraft waiting behind me and I panicked - took off without completing a whole section of my checks. I learned a lot from that - namely do not rush and get it wrong.
But what if this had been a solo student? Mr Pitts wasn't to know there was an instructor on board. Already feeling pressurised because of the busy circuit, the student enters the runway, backtracks (rightly or wrongly), turns, applies full power without looking ahead properly (he is trying to save time remember) and then suddenly finds himself taking off alongside another aircraft who entered the runway without telling anyone."
Excellent points which the OP could have resolved by calling the Pitts through, agreeing to a call from the Pitts indicating his action or being relived that the Pitts took the initiative and went ahead.