Problems because the curriculum of Khan academy (KA) is not equal to the Finnish academy. Students started doing KA:s section 'Exponent's and Radicals'. Problem: after some time it turned out that some question were too difficult compared to what we have in our Finnish textbook.
Lesson learned: teacher must do the Khan-practice in advance.
If there is too big a mismatch between the Finnish curriculum and KA one must do the exercises from the Finnish textbook (do it the traditional way).