If there is a course specifically for the students you don't want to access forums, why don't you just not put forums in the course? Maybe I'm not understanding what you're doing. I don't understand the can't use courses comment.
If they all do all three, the enrollment is irrelevant for Rick's suggestion--in each relevant course, create groups: those who can see the forums and those who can't. Go to the forum settings and restrict access (hide) from group that is not allowed to participate. This should be possible in 2.7 and is definitely possible in 3.x. I'm pretty sure it worked in 2.8 but I don't remember 2.5 well enough...since it is right in between...maybe?
As for disabling IM...maybe you just create a second Student role (this would be a site admin thing) and restrict permissions to use IM...you could also block forum use as well, Then enroll that block of people into Student 2 instead of default student. I don't know how your meta is set up...that might not help if the meta is all of them...If you go for the second student role, you can also disable permissions in category areas.