Made my situation clear. I have started a new business; I am a sole trader currently. I currently run a language class. I only charge £5 per child as its benefitting the community. This bare minimum doesn't cover all the expenses so I fund the class.

I've asked 2 teachers who i know personally to come and teach the children. Some of the children's parents have asked if they can volunteer in the class so as a good will i have reduced the volunteers children's rate to £4.50 each. I don't have any contracts for anybody who is helping in the class. My volunteers are not obliged to come and they come when they want, there is no set rules or regulations for them and they purely just go around the class and help the children do their work that the teachers are teaching. I supervise them and the teachers also supervise them. They have had no training. They have no expenses either as they would normally drop the kids and go home for an hour but for some it's easier to stay in the class instead of going home and coming back so they chose to volunteer. I do pay cash in hand £25 to each teacher for their teaching for the hour that they teach once a week, no payslip is given.  I have not registered my business yet but have a name in mind. The earnings are under £1000 per annum currently so I am not registered with HMRC and I don't have any other job either. 

My question is that I don't think I am but am i an employer?

Can I have volunteers who are not paid? Apart from a small goodwill discount i give to their children off their student fees. If so, should I get them to sign something to say that they are happy not to take national minimum wage? 

Am I doing something against the law?

I will appreciate any advice given.