I do not think there is a canon expanation, but there are several factors to take into account to explain the fact that class 3-3 is still open:
- The society believes the curse to be a superstition. There is no scientific proof of the curse existing (even the members of the class have a hard time believing it with people BEING MURDERED in front of hem), and the very curse has effects to hide its occurence (modifying memories and written records).
- Schools in Japan have to meet demand. The school board for their town might have determined that there must be three third year classes because of the population size in the neighborhood. Removing one class would mean transfering kids to schools further away, and would make the school administration seem lazy (almost a sin in Japanese culture). Remember kids walk to shcool in Japan, so they usually attend school near their homes. For further reading into the japanese school system, click here.
These boards are responsible for making recommendations on teacher appointments to the prefectural board of education, choosing textbooks from the MEXT-approved list, conducting in-service teacher and staff professional development, and overseeing the day-to-day operations of primary and lower secondary schools.
- The curse might be self-sustaining. A curse that can change written material, delete people's memories could very well act every time someone in charge thinks of closing the class and modify their heart about it. You might even think of it as having some sinister intelligence.
From our outsider perspecive, watching the story from a 3-rd person omniscient narrator
, it is easy to reach those logical conclusions, but to the limited awareness of the characters (specially the stakeholders that are unaffected by the curse, or only receive the news after a time, already filtered down) it is almost impossible.
Also there is no evidence that making the school have only 2 classes wouldn't make the curse move to class 3-2 instead. Who said it was bound to the THIRD class of the third year and not the last one in the rooster.