The free trial does not commit you to buying anything. When the 90 are over, it will just stop working.
If you qualify for the student discount, "ProApps for Education", you get a sweet deal, with all the pro apps for less than what FCP alone would cost.
If the trial works fine for you, then you can buy at any time. Also: the purchase will be for you to use in all macs (present and future) that you own and control, even after you cease being a student. It gives you all the perks the separate purchase would.