1. Go to emails->find the email named "[{{{site.name}}}] Activate your account"
Edit it and remove the "{{{activate.url}}}" text from it.
This way an activation mail will be sent to the user but there won't be any url to activate it.
THE ADMIN will need to go onto the backend of the site and activate the user manually.
2.
Go to wp-admin->WPLMS->buddypress->redirect page on user activation.
But for this to work, You need to setup custom registration form from the wp-admin->lms settings->registration.
NOTE : You can also set the page for login redirection for both students and instructors.