You can make them higher by doing sit up or crunches. Every jump comes from your abs. The more muscles you have in your core area, the higher the jumps. And squats for your leg muscles can help also.

Crunches and abs are big. As are calf exercises where you stand on a stair and dip your heels below the stair and push up and repeat. Here are a couple more exercises that are aimed holistically at the entire jumping motion and muscle groups. But like any set of new exercises, take it easy at first and focus on your form before you increase weight or number of reps.

http://www.coreperformance.com/knowledge/workouts/increase-your-vertical.html

I hope that helps and good luck!

the most effective way to make your jump higher is through plyometrics. There are number of drills and exercises to help increase your vertical. Yes you are right that strong abs will help you increase your jump but it is only minimal..in the end still your leg and calf muscles and your maximal strength are the core ingredients in making your jump higher. For private lessons anyone can contact me at changlawrence@ymail.com