Sorry to tell you - but you probably blew it.
IF she was seriously interested and a newb to sugar, you correct response was to say something like: "Completely understand how important your education is. Stay focused on getting an A, please. Maybe after the test we can celebrate your great results!" IF she is serious, she will appreciate your support. After all, that type of mentoring and support from a Daddy is part of what the newer SB's are expecting. This will separate you from the guys who just want to dip into pussy. Now, if she doesn't follow up or reply to you next day message of "Hi. How did it go?" (Note: do not mention rescheduling - let her bring it up), then she's probably moving on to a bigger wallet/better looking guy.
IF she was just looking for time to shop her pussy around to other SD's and only had you on her B or C list, then nothing you could have said would make her more likely to actually meet.
Always remember these communication guidelines:
1. Be respectful supportive and empathetic that she's trying to make a better life(style).
2. You are in no hurry to fuck her as you (also) want to build a long term arrangement. (Even if you are desperate to fuck her and will probably dump her after.)
3. Never lose your temper or let her motivate a negative reaction. You are in control of your time, attention and money. Bad news from her drives a thoughtful response from you - not a complaint, whine, criticism or rant.
4. Once you have moved communication off the site, do not return unless she initiates or her other channel is not working (i.e.: you get a "number no longer working" message). If you two are talking offline, do not ever mention you see her still active on the site. It means YOU are still active on the site and will make her think you are looking to bang multiple chicks - which you are.
Remember that she is as motivated to get you on "her side" as you are to bang her so hard she won't even remember her "real" daddy. She wants to THINK you are looking for one, great SB, as she has told you she is looking for one, great SD. Don't derail that train of thought. Ultimately, YOU can always decide to drop her at any time YOU want. But there are several past posts on here from me and others who have left the door open only to see a hot POT SB re-engage weeks or even months later. So don't burn any bridges.