my old boss is making it seem like I’m stealing the account from him
To whom is he conveying that, and in which terms is he doing so? If (1) it is to the school, (2) via false statements of fact, and (3) his misrepresentations deter the school from hiring you, your old boss might be liable for defamation with special damages.
he’s saying I can’t work for them because he did business with them
and that I can’t work with them for a year because I used to work for
The only way it would be true is if you agreed to a non-compete clause. Usually that is done in writing (via email or by signing a contract). The employer can always allege that the agreement was verbal, although that is hard to prove.
Based on the limited information you provide, a civil action in court (or grievance in the corresponding Labor administrative agency) seems premature because it is unclear whether the old boss is factually hindering your employment prospects at the school already.
For the time being, you may want to (1) request your old boss a copy of the contract he alleges you signed; (2) secure written evidence of the school's interest in hiring you; and (3) establish the reason why the employer lost the account (I presume you mean "the contract") with the school. As a precaution against defamatory falsehoods, make sure that henceforth all your communications with the old boss/employer are in writing (as opposed to oral).
From personal experience, I know that an unethical employer can (and does) resort to defamatory falsehoods in his attempt to remedy his inability to persuade his employee to decline an offer of employment elsewhere ... in my case, coincidentally, a school (the University of Michigan) for which I had performed work on behalf of the employer.
I sued my former employer in pro per (meaning that I represented myself in court, without an attorney), and currently this matter is pending review in the U.S. Supreme Court. But litigation is difficult and very frustrating because many judges are tremendously corrupt. Thus, hopefully your employment prospect moves forward without having to bring legal action against the old boss for any unfair and deceptive acts of his.