There are a lot of words in the letter that are either spelt correctly and I doubt a child would spell them all correctly, like any, doesn't, because for example. Or words that are spelt incorrectly but not in the way that children would actually misspell them, like schol for example. As a pp pointed out, children in England (and I presume the rest of the UK) spell using phonics at 6 years old. So 'any' is usually spelt as 'eny' and 'because' is very frequently spelt 'becuz' or 'becoz/becos'.
Also the handwriting doesn't have a childlike quality. I've seen 5/6 year olds with neater handwriting than on this letter but it's always childlike. Also the tail on y seems off. It doesn't look like the sort of tail a young child would do. Also the handwriting is slanting in places and 6 year olds generally write very upright because they're concentrating and trying to do it the way they learn in school.
I also question the use of 'because' and 'and' in the same sentence. That seems a fairly mature way of writing. There are also no words missed out in the letter and it's very fluent for a 6 year old. 6 year olds often miss words out in sentences because their hand and brain aren't working at the same pace. There is no crossing out of words which again suggests someone considerably older than 6.
If you put all that together then it's obviously fake and an adult has written this trying to pass it off as a child.