Yeah, I have a 12 year old daughter, that are going first year in high school. It all depends, as 1000 baht can be fine, as 2000 baht or more can be fine too. I don´t know your situation regarding school. My daughter goes in private school, and have to buy lunch everyday. Not much, but ends up with 25-40 baht a day, and maybe she want a juice or a tea/coffee from the school shop during the day. That might be 20-25 baht more. After school, I can be a little late sometimes, but she don´t mind as many of her friends are there after school as well. Just outside the school gates, they have a very popular and modern café/eatery. That might be another 30-40 baht. As we can see, the day can total around a 100 baht.
Just giving her a 1000 will not cover much in our situation. I do give both my younger daughter and the high school one a 1000 baht each per month. After that, I send them with daily money for things they buy in school. For the older one it usually total 1000 + 2000. For the younger one she have food included in school, so that totals 1000 + 1000 as she buy something in her school shop, a private school where they also have a café that are ran by the school. So about 100 baht extra a day for high school and about 40-50 baht extra for the younger one.