In many secondary schools, lessons are 50 minutes, so pupils have 30 lessons per week.
As an NQT, I teach 24 lessons, but next year I'll have 27. Aargh!
It is the case that some lessons can be reused, but it doesn't happen as much as I'd like.
I have 2 Year 7 classes, and I do reuse lessons for them (just swapping things around a bit depending on how the last lesson went as one group is starting to pull ahead of the other a bit...), but in the other year groups this isn't possible because my classes are setted.
In Y8, I teach Set 1 and Set 4. In Y9 I teach beginners Spanish, bottom set French, and a bit of top set French. In Y10 I teach top set GCSE and bottom set Foundation Certificate, so the gap is huge and I can hardly ever reuse anything even where groups are doing the same topics.