The official Log Books of Victorian schools have many entries which show that infectious diseases were very common and often very dangerous. Diptheria and scarlet fever were two of the worst illnesses, and they often forced schools to close altogether until the danger had passed.
Sadly, entries like these appear in almost all school records of the time. They are just three days apart in the diary of Llanbedr National School for 1883 :

19th February - "The child Mary Ann Jones, Age 10 years in the IV Standard [fourth class] died on Saturday night."
This poor little girl had been taken ill on a Tuesday and was dead by Saturday.
There are more examples of the effect of illness and epidemics on the early schools on the next page...

More about the dangers of disease...