i always get confused as well. xD
but yeah. she would turn 17 and still be in her sixth year.
i'm always making a list like this one:
1st year: character is 11 and turns 12
2nd year: char is 12 and turns 13
3rd year: 13 -> 14
4th year: 14 -> 15
5th year: 15 -> 16
6th year: 16 -> 17
7th year: 17 -> 18
does that help? ^^