GENERAL BAG DRILLS:
Simply "working" the heavy Bag - This is the one that gets done the most. Let the bag swing naturally from your strikes and move around it. Throw different combo's and try to stay relaxed and in balance while hitting hard. You can emphasize different things or strikes easily. This works best for rounds or time.
50's or 100's - Just throw 50 or one hundred of a strike or combination on the bag. Power and form are the buzz words. Go for mastery of the technique. Taking an extra breath between reps does not matter if it means that all the strikes are quality. This could be something like "100 hard round house kicks", or it could be "50 of double jab, step off line". Just get the practice in.
I have heard this called "hammers" or "cannonballs"- This works for Thai pads as well as a heavy bag. Get in your fighting stance. For whichever side is back, throw a hard straight with no telegraph. Go back to guard. Then a hard elbow. Back to guard. A hard roundhouse kick. Back to guard. A hard knee. Back to guard. That is 1. You can also do it with the lead sid by using a hook and switch kicks/knees. This is about power, plain and simple. Emphasis is on hitting hard from your fighting stance. THIS IS NOT A COMBINATION. It may feel awkward. That is fine. It seems to work better with 3 minute rounds.
Strike and Sprawl drills- Any combination, followed by a sprawl, followed by standing up and throwing a punch or a knee. Repeat. It can be done at a medium pace for rounds. Or non stop for short bursts, working up to a minute. If you do it non-stop it fries your lungs/wind.
Since you are going to be operating a bit behind the eight ball in terms of practice time (since you can only get formal coaching 2 X a week) I recommend three points of emphasis for these extra workouts.
1.) Working on things YOU do poorly- If you have trouble with something in class, or you repeatedly get caught or cannot perform in a situation you are going to have to do what you can to get extra work on your own. The option of just doing what you enjoy, or what your coach/teacher has you do in class is not going to fly in your case. You will not have the "in class" time to even hope that your weaknesses get addressed normally.
You have 2 years experience so you likely have some idea about your weak points. If they can at all be worked on your own, do so. The obvious weakpoint is going to be "NEW STUFF". Most everyone is bad at something they try for the first time. At 2 times a week there may be a lot of calender time between a "new" technique and when you see it again in the class. Thus, simply reviewing what you did in class for 20 minutes or so becomes a very good learning tool. Even if between shadow boxing, shadow wrestling, and the heavy bag you can only "sort of" do it, do it.
2) Working on things we all need to practice - There are certain movements and techniques that are absolutely fundamental to martial arts/mma/fighting. A big part of training is simply working these movements over, and over. You can absolutely do this on your own. Because of your situation you must do this on your own.
I am of the mind everyone needs to be able to jab. I did not say every boxer. I said everyone. In fact I will go the route of saying if you only have one punch, make it the jab. Working a variety of drills that train the jab at home will help you. It is usually the easiest blow to land. Even if it only breaks the other guys rhythm it will do a lot to keep you out of trouble. Presumably you are going to be sparring with people who get more training time, and thus more experience, than you. This means that you cannot afford to let them dictate the terms of the engagement. Failing that, simply covering and throwing jabs will allow you space to move and survive some rounds that you would not have otherwise. So, practice the jab. Practice the double jab. Work the jab as a way to set other techniques up.
The Cannonball drill practices the 8 strikes that damn near every MMA fighter throws/trains. For grappling I submit that you should be shrimping/elbow escaping, bridge and rolling, hip heisting, bridging and turning, going from turtle to on your back and then returning without leaving space at our hips or neck, and standing up. You can practice all of these by yourself.
3) Work on your strong points - I know eliminating weakness gets a lot of press. It should. In your case I am going to argue that it is just as important, maybe more important, to make your strengths even stronger. With limited training time and other priorities, school, you are always going to have weaknesses and things that need work. You need to have some things that work well enough to give you an A game. So, whatever techniques already work for you, practice the hell out of 'em. Find ways to use them more often. If your right straight is your best punch. Stand, move, and fight in a way that gives you chances to land it. Practice combos that have a right straight in them. Do this with whatever you are good at, or whatever seems to work well.
The reason I am writing all of this is that if you have to train very part time, you are likely going to get tooled on some. I am not trying to be condescending. You are going to get tapped when rolling and hit when sparring. The two things that will minimize your injuries, and make training fun enough that you stick with it are defense/surviving and being able to pull off some techniques. This way you will tap less, and have "something" to try. For stand up, a good guard, and a good jab will make you harder to "beat". On the mat good posture and defensive positioning will make you tougher to tap. This means you can have more fun, and learn more while you are training.
You can make great strides at 2 x plus a week and still do well in school, but the training has to not suck so bad/be enjoyable enough that you stick to it.