I don't see a problem with it. AFAICR we had other people in our school recruiting, why not the army? When's all said and done its still a job. At my school it was heavily linked to the CCF. There was no uproar from the students. If you want to do it, you did and if you didn't, you didn't.
I'd go as far to say we should encourage MOD recruitment in schools, (along with other essential emergency services), because low recruitment would ultimately mean a return to national service to keep the numbers up.