From what I can tell you would not be eligible for any except perhaps the medical exemption (depending on your specific medical condition/s). Although you are a full time student you are not under 19 so can't be exempt due to that. Although you claim PIP you aren't claiming any income related benefits and are earning your own money so I don't believe you qualify for exemption under any of those reasons.
If you are eligible for medical (or any other) exemption I believe you may be able to back track that a certain amount meaning you could be refunded the amount you paid fro prescriptions in that time.
It appears though that unless you have one of those specific conditions that pre-payment is your best bet.