The people in Congress don't understand the basics of Bitcoin, their tweets are pieces of evidence of it. But if the kid is really interested in Bitcoin, nothing stops them from reading or watching youtube discussions about it.
I wonder what would be the ground for this kids to get interested in Bitcoin? Aside from watching animated movie or cartoons that discuss about Bitcoin, I do not see any possible interest this kids may have. Yes they may know Bitcoin by name but I doubt they will dig deeper than what they have watched in animated film or clip. Unless older people drive this kids into learning more about Bitcoin, kids will always take interest in fun activities and will often deviates from serious things since their nature at that age is to explore and have fun. I doubt kids at that age finds fun in exploring serious matter about Bitcoin.
I don't think it's appropriate to teach children before the age of 10 about Bitcoin and things related to trading and the like, there are much more important things for children of this age.
Also, I do not know what the legal position is towards teaching minor children things that may be prohibited in many countries and involve risks, such as Bitcoin and trading.
Don't forget the parents' attitude towards this as well, I'm sure many parents of young students will not like this.
But that's if it's about investment. Teaching kids what money is, how it works, and how it can be used is IMO totally normal and can be done at a very young age. Kids need to understand money, what it can buy and what reasonable spending it, and getting some basic notions of different kinds of money (including Bitcoin) seems like a very neutral and age-appropriate topic to me.
Oh, the op mentioned that it was a maths lesson, and I've remembered that I teach the basics of maths and logic to a 7-year-old, and one of the official books for the subject actually has tasks that work with money denominations (like dollars and cents), how they add up etc. So money can already be in the program, and mentioning Bitcoin is just a reasonable addition from a teacher in such a case.
Yeah this is also another reason exchanges and trading platform allow 18+ people? And reason why kids needs parental consent when parent used their kids name for investment? Because kids are not supposed to deal with investment at that early age. They are incapable of weighing pros and cons of the complexity of trades and investment. Adding and subtracting with numbers are way different in equating the possibility of profit in a trade. Yes mentioning Bitcoin as an example or as unit of measures in an example mathematical computation is ok, but teaching kids about economics of Bitcoin at such young age will only burden them.