Teaching - either independently or assisting your instructor - is, IMHO, a big part of learning; after all, what better way to check your understanding than to explain to someone else? My students teach each other all the time.
As far as being
required to start a school or not be considered "serious"... I don't think I agree with that. Not everyone can be a school or club owner, for reasons related to their life circumstances.
I started instructing because I felt it was the only way I could repay my sahbum for the time and effort he put into teaching me - I continued teaching because I greatly enjoy it. When I decided I wanted to instruct, I talked to my sahbum about it, and he sat down with me and discussed the pros and cons, and made damn sure I knew what I was getting into - not because he wanted to dissuade me, but because he wanted to be sure I had a realistic understanding of what becoming an instructor meant, that I understood the responsibility I was taking on.
I was once offered a job with a local mcdojo chain; the owner of the chain showed up at a teachers' job fair I was attending 13 or 14 years ago. He had a contract-for-rank system - pay $X/month for Y months, show up Y times/month, and you are guaranteed Z rank - and somehow (according to what he told me that day) this system wasn't generating students who made good instructors (go figure, right? :idunno: - :lol

, so he decided to hire teachers (who, presumably, already knew how to teach - although as a school teacher I will say that, while there are similarities, there are also some major differences), pay them ~50% over minimum wage to train in TKD and/or Karate 30 hours/week for 3 months, test them at the end of that time, assign them a rank - anywhere from green to black - and help them set up a franchise in his chain, which would pay salary + commission, commission to be based on the number of paying students and the length of the contract. He was quite excited to hear I was already a black belt in TKD... but I couldn't go through with the interview; I called and canceled, because my personal integrity wouldn't let me get into that type of situation.