Sooner or later you will be asked to teach. At some point the class size, the distribution of junior and senior belts, and your knowledge, (relative to someone else's), will be just right - and you will be asked to teach. Eventually we all accept the realization that as martial arts students, one of our responsibilities is to mentor others.
I was a Green Belt the first time I was asked to help a new White Belt with her sparring techniques. Truth be told, I wasn't very good, but relative to the White Belt, I was sufficiently advanced.
At first thought, you might question the wisdom of this, or at least the financial ethics. After all, you have paid a tidy sum to learn self defense from a qualified instructor, probably a Black Belt, possibly a Master. Now you find yourself providing instruction to someone a few steps junior to yourself. Or think about their perspective; they are under the tutelage of an amateur (you), a person who has merely been allowed to peek one page further in the training manual.
On second thought, however, this is not all that different from when you were in college and you asked the TA (Teacher's Assistant) for remedial enlightenment. In fact, there is an obvious logic to getting help from a fellow practitioner who has more recently traversed the learning gap than your Master - a person who hasn't been where you are for twenty years.
There are three reasons why students teaching students is a good idea:
More recent learning experience. As I mentioned above, Masters have not experienced the learning challenge for a long time. We somethings underestimate the value of empathy - knowing how someone else feels - in the learning environment. Sometimes a person who has just recently learned a new skill is better able to pass it along because they remember how to "climb the hill"
Expands the one-on-one instruction. Everyone loves the special attention that comes from one-on-one instruction. Unless you are the school's only student - the Master simply cannot give everyone his full attention. By matching advanced students with juniors, the newbies get much needed personal mentoring. I should point out here that when I pair up students like this, I always stay in the training hall to monitor and ensure proper instruction.
Reinforces the knowledge, a.k.a. deliberate practice. I cannot emphasize this enough; nothing improves your capability and understanding more than teaching. You become a better student when you are made to teach. You have to think through the information more deeply, you have to answer questions you never thought to ask, and you need to demonstrate with deliberate precision.
So when you are asked to help out - consider it recognition that you are progressing, and think of it as an opportunity to really focus on what you have learned. Trust me, you will benefit every bit as much as your 'student.'