I agree with city flyer's comments above.
I would also add that in the case of CTC (OAA may have similar arrangements) the bond monies are paid in monthly instalments. There is a payment prior to the commencement of the ground school and there is a further payment for the foundation course.
These payments are made in advance of the training received so to that degree there is always an element of "risk." However the payments whilst keeping the training organisation in credit, are in my opinion kept at a reasonable level and only at a short term period (month or two) in advance of the services provided.
There are also some elements of "bond protection" provided as a part of the course and the details of these can be found on the FTO's website.