Does there have to be a meaning behind everything? I just assumed the school taught kids kindergarden through high school or whatever they call it in the UK.
They couldn't afford that many kids in the cast (the amount they already have is driving their weekly nut up like crazy) so they just chose to cast them as adults because it doesn't really make that much of a difference.
Anything regarding shows stated by this account is an attempt to convey opinion and not fact.
A variety of reasons both symbolic and realistic. Realistically, adults can perform 8 shows a week and endure the work that comes with it. They can also perform for long amounts of time without needing to be replaced. Matilda probably has to swap out the kids quite a bit. It's easier to teach a group of kids and a group of adults a number than two groups of kids. It also creates a bigger and more varied sound on stage. I imagine less kids cuts down on the chaos too. Matilda seems to need quite a few adult ensemble members...I'm sure the "older kids" play several other small roles.
It also reflects not being able to cast actual Kindergarten-aged kids. Since they're using 10 to 13 year olds as 5 to 7 year olds, they decided to cast adults as the 10 to 13 year olds.
Just for the record, two of the current Matildas are 8 years old.
But yes, your basic premise in principle is correct. Plus, I think the vocals of the group kids sounds a bit richer when you add the adult voices to the mix.