With regard to breaking boards, Tae Kwon Do is famous the world over. But, teaching students to break a board can be the hardest thing to do. Surface, in this context, has two meanings. The most obvious meaning is the surface of the board.
When breaking a board, it is important that you strike through the surface of the board, usually somewhere behind the back of it. Hitting on the surface has no effect on the board i.e. it will not break! But, this is where the second meaning comes into it.
In order to successfully break the board, it requires a certain amount of confidence, something that cannot be taught. On the surface, a student might look confident. But, underneath, they might doubt their abilities and not commit to the strike. It’s a bit catch-22, they get confidence by breaking the board, but until they break the board, they are not confident about success.
All you can do, is show them the techniques and let them have a go!