The school holds a job fair, and students who do not participate in it will be disqualified and will not be recommended for employment. Does the school have the right to do this? Is it reasonable?

Legally speaking, the school doesn't! But the school is also compulsory employment, so you will generally be forced to sign an employment agreement! I think you should look at your situation. Don't sign an employment agreement if you want to take the civil service exam! With the notice of choosing a job, the civil service job fair will be a fresh graduate!