As above, you are entitled to receive compensation under a wide range of circumstances. Fill out this form, and keep records on how many students are assigned to your class and for how long.
Many administrators routinely break up classes instead of calling for substitutes because it saves them money. Certainly schools need more resources, and it’s always good to save where one can, but it shouldn’t come at the expense of loading up “receiving” classes, which helps no one.
Providing compensation to teachers who must do additional work is meant to be a deterrent to this practice.