(1) The demonstration must be tested beforehand so that a fuss doesn't have to be made over the block sliding too far or the hanging mass being too large or small.
(2) The block should be weighed before class also, watching the block being weighed won't teach the students much.
(3) Measuring mu (the coefficient of sliding friction) accurately. This may need to be measured in the class itself, as changes in humidity and handling can change the coefficient of friction. In the case where this constant is the unknown, previous measurement and a little hand-waving may suffice.
(4) The table. It may not be exactly level, and this could throw off the distance the block slides. Over the length of a board that one could consider typical the effect could be significant. Even side-to-side unevenness can create an extra component of force. A level and three supports would suffice to make the board statically determinate.
(5) When calculating the coefficient of sliding friction and doing the demonstration, be sure that the same sides of the block and board are in contact. The coefficient of friction can be drastically changed if the block or board are turned upside-down.