High school in the US == 6th Form in the UK.
The thing is, while the US tends to have the Elementary/Middle or Junior High/(Senior) High split in most places, the grade levels aren't the same.
I've seen (and this may not be exhaustive) the splits with the following grade levels:
1-5/6-8/9-12
1-6/7-8/9-12
1-6/7-9/10-12