If on a French contract, some months you may make 1200 (starting salary), but some months you may make 600/500. Tough months are especially January and August. December is a bit slow as well for some teachers. (nb: this is probably very similar in most business English companies. In your contract a minimum amount of hours to be paid is stated, however it is a very low minimum). There is also no holiday pay for your first year at BTL (although technically the holiday pay is paid to you in August after a year of working... I think... In any case, these only helps to alleviate the very low pay you will receive in August because you don't have enough classes to teach)
If on an English contract (hired in England), you have a fixed salary, however your life is in BTL's hands: you basically can't refuse any classes. And since BTL has to pay you a fixed salary, they like to give you as many classes as possible, sometimes over what is stated in your contract. They also sometimes schedule classes outside of the hours stated on your contract (ie until 7 or 8 pm whereas in the contract it says till 6pm)
Specialised English classes: Some classes can be specialised for Finance, Advertising etc. BTL receives a premium for these classes but professors do not. More work for the same pay.
Like most language schools in France, there is no sick pay at BTL.