Yes, you should.
Nobody hashes (passwords are hashed, not encrypted) passwords because they KNOW that they have an SQL injection vulnerability. It's a second layer of defense.
And it's needed because you just can't be sure that you don't have an SQL injection. Or that a library you use doesn't have any. Or that there are no other vulnerabilities, such as XSS (if you have a database export option), LFI (if your database is accessible from remote), code execution, badly written backup script, etc via which an attacker can get access to the database.
I'm not quite sure what "developing a school" means, but if you have actual users, and an attack does take place, it will be very bad publicity if your passwords are unhashed, and there might even be some liability on your part.
That are the reasons for hashing. What are the ones against? I mean, it's easy enough, so just do it.