What's the Name of That Book??? discussion

I thought this was a book by Pete Johnson, but I've gone through a lot of lists of his books and can't find what I'm looking for, so maybe I'm wrong or it's one of his more obscure ones.

It's a book and a sequel which both centre on the same group of three teenagers, a boy and two girls. One of the girls is called Mikki and I think she is the "weird kid" of the group although none of them are particularly popular. The books are just about their day-to-day lives, school, friendships etc. Specific things that I remember:

Mikki has a "special tree" in the grounds of their school that she likes to sit under, and anyone who disturbs her there gets their head bitten off.

One of the girls is getting ready for a party and thinks about what to wear. She asks herself "Who to be, who to be?" and decides she is going as "grunge me".

(view spoiler)[ I think Mikki eventually dies in one of the books, possibly by suicide, and the other two have to come to terms with her death (hide spoiler)]

I read it in the early-mid 90s and think it was quite new at the time. Any help appreciated.

Just to add: definitely a British book by a British author.

Maybe Writing in Martian by Andrew Matthews? Its sequel appears to be Seeing in Moonlight

OK, they were not by the above Andrew Matthews, but this one: Andrew Matthews I'm moving them and some other mis-attributted books.

Ayshe, you've done it again. Thank you so much. How did you find these? tell me your secrets!

Great :) Just luck, searching in Google books for "two girls" school Mikki sequel and years range 1980-2000 brought up: This is a sequel to Writing in Martian it describes the friendship of Mikki, Bron and Wayne... "The School Librarian" and it seemed like it could be your book.

Thanks, I tried google and google books myself, but I guess you have the magic touch!