It's great that you're offering a mentorship program, but I would have been one of those students complaining. In a business setting I understand the need to optimize onboarding, but in an educational setting I think inclusiveness should rank higher.

In our student organizations (at university) we've settled on Teams, which is what's available through our university accounts. I'd guess most universities have an equivalent, but let me know if I'm wrong. This has the benefit of requiring no sign-up, and separating work and personal life.

If you're talking about HS students, then I'd be surprised to hear that they're unwilling to install your preferred chat app to join the program.