When in the bottom position, standing up is the easiest, quickest, most proficient way to escape. It should be the first thing you try every single time you hear that whistle or whenever you feel a lack of control from the top guy. You'll be working on this for the rest of your wrestling career. It is used in every single level of wrestling. The more you drill this, the better you'll get at it. If you get good enough, you'll never need any other tactic from bottom.
Look to get wrist control on the way up or when on your feet to avoid being lifted. Keep yourself in good position and stay up for as long as you can. After five seconds the referee should be looking to call the top guy for stalling.
Also look out for the top guy dropping to your leg. If you feel a sudden lack of control and your opponent is lowering his elevation, you must face, sprawl, and drop your hips as fast as possible so you can break his grip before he has one.