Hmm; I assume you tried testing with any and all variations of the school names you could think of, as well? If you did, without sucess, you will need to submit the Yahoo! Account Recovery Form, using the following link:https://io.help.yahoo.com/contact/index?locale=en_US&y=PROD_ACCT&page=contact
In that page, under "Topic", choose "Password XXXXX sign-in". Next, select "Unable to sign-in".
Now, please locate the envelope icon labeled "Email". Click on that envelope icon to bring up the Account Recovery Form.
In this form, you need to fill out as much information as you can to help Yahoo! verify that you are the rightful owner of the account that you are trying to recover. The more accurate information that you provide in the form, the greater are your chances of getting your account recovered accordingly. Once you have submitted the form, Yahoo! will review the provided info, and get back to you at the alternate email address you provided, within a week - and assist you with resetting the password XXXXX
I know this process might be inconvenient for you, since you will not able to regain access to your account immediately. However, I just want to be honest with you and let you know that this might just be the only option you have for recovering your account itself. Good luck, and do let me know if there's anything else I could assist you with. Thank you!