Look buddy.
You can either do it the RIGHT way or the WRONG way.
It is your choice. Letting her ask for exclusivity is a 100% Money Back Guarentee that she wants you and no-one else.
Some HS girls believe that it is the GUY'S job to ask the girl out. Personally, that is not my style. However, you can still be exclusive with this type of girl if you are not the one that asks for exclusivity.
Either way, you can get the girl to be exclusive with you. There is just a RIGHT way (100% Money Back Guarentee that she wants to be exclusive with you and you only) and there is a WRONG way (you ask her to be your girlfriend and she becomes exclusive with you, but there are no guarentees that it will last) to do it. Both ways will get you exclusivity. One is just BETTER than the other.
The decision is yours...
Do as you'd like.