Sharing allocated support hours

[spearhead]

My son is nearly 9 and has 20 hours support a week for his Aspergers. This morning, I heard his class teacher talk to another parent about something, and she stated she would talk to MrsX about it and they could plan something. Now my problem is that MrsX is employed for my son, she was not at the school until my son got his hours, and only works those allocated hours. So, is this something that the school can do, use my sons support time for other students?
Thanks for any help anyone can give, I will worry about this all day now.

Could teacher asking Mrs X advice? Or That Mrs X saw something happen teacher needs to check up on?
Does your son always have 1:1 or does he do small group work & child could join the group?
It's not always that your child would lose time( although I know that lots of schools might use it for other children).