Have to = must/it is obligatory for the boys to as at least one girl. Should = is a word we use when we are giving advice.

I think there's a slight misunderstanding of terms here, but I'll try to answer what you want to know. "The boys have to ask at least one girl." This is required. "The girls should ask at least one boy." This is suggested. "Ask at least one boy/girl." This is imperative.

Unkraut, I have n't seen you saying "Thank you".