@Istvan ********
If you're questioned why/how you're on holiday during term-time, you'd definitely need a letter authorising you to be away from class. I didn't say it's something that always checked, but it has been reported as an issue for some people over the years so it absolutely is "a thing". With respect, simply because you haven't heard of something does mean it doesn't exist. Like they say, something isn't ever a prone until it's a problem. My school didn't even tell me I needed a letter cancelling my ED visa, and I left Thailand and returned without any questions from the IO but at my next extension the IO insisted on getting the cancellation letter. Just shows that there's not always consistency amongst IOs (or schools for that matter)