Assuming you have a user profiles sheet, separate your clients and your students by something like a role column like Pablo suggested.
When you add a new student, make sure you add a client ID, or whatever is unique from the client, in the student info. You will also pass that info to the submitted hours of each student so you can filter the hours list by that info.
Same approach as above, your projects table should have something from the company/client that is unique. Seeing you’re using a free version, try adapting this to add multiple students to a project to save rows.
Same as above, see the Trebuchet thread.
There should be no limits to this using the approach above.