I am currently employed as an administrator at a private academic instituiton. My supervisors have been aware for the past 8 months that I am leaving my position, and they've hired my replacement, who is currently in training. I am leaving on good terms and will be on the payroll until early September.

In May I had my annual performance evaluation and recieved the highest rating possible.

Everyone at the institution who received this high rating received a 3.5% raise effective July 1.

The institution is denying me a raise because they say it is simply not standard to award a raise to someone who is leaving.

Is it legal to deny me a raise that was issued to everyone else across the board?

Unless you have an employment contract guaranteeing you a raise every July 1st, you have no legal right to ever receive one. Thus, they are at the discretion of the employer.

Unless you are being singled out to not receive a raise on the basis of a protected characteristic (gender, race, age, and so forth), their policy is not discriminatory. "Employee who is about to leave" is not a protected characteristic.

It is perfectly normal and legal to not give a raise to someone who is leaving.