Spoilers for Assassination Classroom's ending.
In the final battle,
Kayano is killed, but Koro Sensei manages to repair most of the damage by using the somatic cells he collected from her in the same instant.
Koro Sensei mentions that not all cells can be repaired, so instead he filled the gaps with his mucus and in a few days Kayano's cells should regenerate and replace the mucus on their own.
But this occurs a little before the Heaven's Spear laser strikes, and we know that all the students were hit by it. This would mean that Koro Sensei's mucus would have evaporated, opening the gaps again.
Is there any indication that Kayano's health was affected by the laser strike? Of course I doubt that the gaps would be enough to kill her or cause any major damage anyway, but this seems like a pretty obvious situation to consider.
A possible explanation is that the mucus is unaffected by the laser, but I doubt it because the engineers were complaining that the strike would leave no sample material they could analyze (and I imagine that the mucus counts as sample material).