Are you sure its not just a reminder and you were informed of the date at the start of the year? Are any other INSET days on the website/social media?
Ridiculous short notice if not, considering that INSET days are set by the LEA
Not here they are not. Usually schools are able to set their own dates. these are, however, normally set well in advance with a lot of noice given, ime.
Schools usually start back on the Tuesday of each new term.
Also not the case ime. Schools only have 5 INSET days a year. IME they are not always set the for Monday after each half term/holiday. Often they are on other days of the week, esp a Friday, or some taken in 2s/3s at the start or end of the year.
Should I take him anyway as Teachers will be there?
They won't let him stay. They will send you away with him. Infact, the gates may be locked - ours are on INSET quite often, as staff can access with the code.
Have you been in touch with other parents? If it is really such late notice, surely there will be many parents in this situation and school may have a huge issue on their hands tomorrow!