Fix three code bugs

[adrian-rpf]

Status

• Closes add issue numbers or delete
• Related to add issue numbers or delete

Points for consideration:

• Security
• Performance

What's changed?

• Project.users: Refactored to correctly fetch student users for multiple schools by grouping user IDs per school and making individual API calls, preventing data loss when projects span different schools.
• SchoolClass.with_teachers: Corrected the scope to filter on class_teachers.teacher_id instead of teachers.id to accurately identify teachers associated with a class, resolving false negatives.
• StudentRemovalService: Modified to only skip student removal if projects exist within the current school and optimized the check using .exists? to prevent unnecessary data loading and ensure students without in-school work can be removed.

Steps to perform after deploying to production

If the production environment requires any extra work after this PR has been deployed detail it here. This could be running a Rake task, a migration, or upgrading a Gem. That kind of thing.

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