K.M.
I agree with everyones comments so far. If it is an issue that only happens at school it is their job to help him through it - it is lazy of them to expect you to make him take a "step back" in training when it may just be the distractions of the school environment. Asking them to remind him is not a big deal for them.
Also is there a good friend of his they could pair him up with to go at the same time? This helped my son during a stage when he just didn't feel like it.