I find that disgraceful! Although, I'd play things a little safe.
Can you speak to the one in charge and get them to do something about this. The one in charge needs to know this is happening, otherwise, what happens if your ill for a week and the other teacher can't prepare for a lesson because they've been copying your lesson plans?!
However, I hope teacher B isn't the one in charge of the tuition centre.
definitely raise this with someone higher. Otherwise, teacher B may as well be giving you some money for using your lesson plans.