MIL would not have let her sons go.
She did not let her 17yo son go on a skiing trip with the school (not so many years ago)
Partly because MIL is so old-fashioned, she would think all the following:
"I didn't need to do things like that when I was young"
"They won't eat regularly"
"They won't keep to their same routine"
"They might be exposed to other boys' bad habits or drugs"
"They won't like it"
All of that is code for, "I'm not enough in control of their routine if they're away that long"; and MIL is a routine-junkie. In an age of Gina Ford adoration, I'm sure many parents are still like MIL even now (lljkk runs away cackling)