Hi there,
My dd has been diagnosed with ASD at 15 - rather late than never! It is quite an adjustment and I find that everything is a struggle. School is being slow in getting an IEP in place and making adjustments that lessen her anxiety.
How demanding / pushy can I be?
With school? Absolutely as pushy as you want/need to be. It really depends on the school, some get around to these things before the diagnosis proper is even in place, some (including, unfortunately for me, the high school I went to as a child) require you as a parent to be an absolute thorn in their backside about it constantly.
They do have an obligation to make adjustments that enable your daughter to access her education as easily as a pupil without ASD (it's right there in the equality act as it applies to education) and they should know it full well, but some can need reminding.
You might find this recent thread a good read https://community.autism.org.uk/f/parents-and-carers/13345/how-to-handle-14yr-old-daughter-s-school-refusal-and-very-low-mood as it concerns another child around your daughter's age who is also struggling with school-based anxiety, which is very common in teenage females on the spectrum; it's often why we get diagnosed in the first place. 
Welcome to the forum!
Emma x
