Now the REALLY IMPORTANT part is that your teen sticks to this timetable

Sticking to a routine and being consistent in their study habits is what will propel your teen forward.
Getting used to studying and doing homework regularly is going to get your teen the grades theyre capable of.
For extra brownie points all round, make a few copies of the timetable and stick it around your house on the fridge, on your teens door, and in their room so that they feel accountable, and are therefore far more likely to stick to their timetable. This is just another thing thats incredibly simple to do, yet can make a huge difference to your teens grades.