I saw similar question on Reddit few years back and I will try to recreate answers as best as I could since I can not find it again for some reason (my google is behaving very strange today)
I don't think school notify muggle parents since they would probably forbid their kids to attend school and all victims were muggle-born Colin Creevey , Justin Finch-Fletchley , Hermione Granger ( not definitive for Penelope Clearwater but she is listed as Muggle-born or half-blood)
Also a quote from Dean Thomas (Harry Potter and the Order of the Phoenix) comes to mind
“My parents are muggles, mate. They don't know nothing about no deaths
at Hogwarts, because I'm not stupid enough to tell them.”
So I am pretty sure kids don't tell their parents what happens at Hogwarts 90% of the time
To answer your question why parents didn't notice anything when they didn't come home for Christmas well I would say that its isn't unusual for kids to stay at Hogwarts over holidays and in middle of Order of Phoenix when Hermione wanted to spend time at Number Twelve, Grimmauld Place she tricked her parents into believing that she will be staying in Hogwarts
Harry Potter: "I thought you were skiing with your mum and dad."
Hermione: "Well, to tell you the truth, skiing's not really my thing. So I've come for Christmas... Anyway, Mum and Dad are a bit disappointed, but I've told them that everyone who's serious about the exams is staying at Hogwarts to study. They want me to do well, they'll understand."
— Hermione telling Harry Potter about changing her plans for the 1995
And about letters. Considering that there is magic in this world it wouldn't be to hard to charm a quill to write a letter every once in a while to parents, besides since parents are muggles I am not sure they would be able to send anything to their kids without use of an owl which they don't have
On the other hand when Ron got poisoned on his seventeenth birthday and when Ginny got taken to Chamber of Secrets their parents were notified and they didn't forbid them to go back to Hogwarts. So it is safe to say that pure-bloods are expecting some sort of trouble and accidents. After all it is a place full of children who are LEARNING to use agic