We all know Maya is desperate for the free tuition in order to get from under her dad's thumb due to conditions he's placed on releasing the money from the cosigned loan they organised. So regardless of the validity of this scenario, we can accept Maya needs it.
For the other girls, depending on the courses they are attending, the average annual tuition cost is 40K. So 4 years of that, 160K+ debt. I don't think Mona needed it so much as decided it was a good way to avoid starting life owing a large student debt.
In the pool scene with the HOTs, Quinn brings it up:
Quinn: "I'm aware that you ladies have heard some rumors about the HOTs, right?"
Maya: "I've heard something about...free...tuition."
Quinn: "Did you hear about that, as well, Mona?"
Mona: "It's been mentioned in whispers."
Quinn: "Ah! The grapevine..."
Quinn: "Seems like there's a HOT among us who can't keep her trap shut."
Quinn: "You know how the saying goes..."
Quinn: "Loose lips, sink ships."
Quinn: "But loose hips, get tips."
Quinn: "So, girls...are you interested?"
Mona: "Free tuition? Sign me up."
Mona: "I'm not passing on that offer."
Quinn: "And you, buttercup?"
Quinn: "Are you in?"
Maya: "What do we have to do, exactly?"
Sage: "All right, sisters and daughters!"
Sage: "Sorry for the delay!"
Mona just jumps on the band wagon because it's offered, not because she has no other way of paying. Her leaving probably has less to do with losing the free tuition and more to do with getting involved in a prostitution ring...
The other girls, Melanie, Sarah, Camila and Riona (as far as we know that's it) are also in it for the cash because it's on offer, not because they can't afford it. All of them could just take out a student load, but the Restaurant business saves them money in the long run.