a standard set is 12 reps but here is something you can also try later on. This works great for squats but because your still young and growing squats may not be so good to do. For large muscle groups like your chest, quads etc. do 6-9 reps but real heavy almost to your max doing 3-4 sets. Now you must stretch before and after every set to get all the benefits and also because if you are like me you will get the craziest cramps in your muscles. Your muscles will feel like you took a pocket knife and cut them apart yourself after the first time you do this.
Its cool that so many young ones are taking to this site for info. More young minds for Thai Bri to warp. Good luck on your test.