Schüler kritisieren auch, dass die Intensivierungsstunden, die zum Üben gedacht seien, auf Grund der Stofffülle für neuen Stoff verwendet werden müssen.
I came up with following translation although I feel something is wrong about it.
Students also criticise the immersion classes, which are intended to be for practice, due to the new material must be applied.
This doesn't make too much sense to me.
Also, what does it say "gedacht seien", not "gedacht sein"? What is the difference between Stoff and Stofffülle?