DD13 is in grade 7. She has a wonderful teacher and great Bnei Akiva madrichot.

The problem is that she has some really nasty group of girls in her class. Bossy and mean girls like I've only read about in books and seen in movies.

DD has learned that it isn't worth befriending these girls and that their nasty comments aren't personal. However, their behaviour really bothers her and fragments the class/shevet.

DD is afraid that their shevet Lag Ba'omer bonfire is going to be ruined. These girls are already picking fights and want to have the bonfire with boys (not accepted where we live).

What route should I take with her? madrichot?

you should speak to the madav. (the madrich mevugar) do you know who it is? our madavit is very involved in those things and meets with the vaadat chinuch abt problems like these. do you know yours? do you want me to ask my friend who is ours who yours is? or to the the comunar/comunarit of bnei akiva.

(I am assuming the madrichim are just older kids in high school also - that is how it is here, i dont think they are capable of dealing with the situation)

Firstly - seems it isn't unusual - dealing with that in Gr7 here (but an ongoing theme that some girls don't have the instinct for nice middot, and as they get older it gets nastier and more painful).
Secondly, I think emunahbutterfly has a good suggestion - go to the top of the chain of pastoral care - I think the magavit is a good person from her description. You need someone who can accurately assess the situation, interact with the kids causing trouble and work with them, support the kids who might be victims, and liaise with the other authority figures in the life of this peer group.
If you don't find that you get success on your own, talk to other mothers and work as a group to approach the leadership. And be persistent with follow-ups and updates on the situation - both giving your own and requesting them for yourself.

Good suggestions which I have already thought of but I'm not sure who is in charge of junior high age. I have to ask my neighbours - they are in charge of youth (high school age).

The madrichot are great and are trying to resolve the issues but the are only 16 and don't have the answers. DD said that last nights "peula" (activity) involved a lot of arguing and tears as they tried to work through this as a group.

These girls are ruining the Lag Ba'Omer plans which is what bothers us most at the moment.

None of my kids do Bnei Akiva, but this kind of thing is unavoidable. Any time you get enough people in a large enough group, stuff like this happens. Think shul politics. Or yishuv politics.

Young people are just more obvious about it.

So if the question is, given that there are people who will insist on behaving badly, how do you not let it ruin your life?

To that question, there are many answers. I'm sure she can come up with some, with her friends, with her madrichot.

I'm so glad that the madrichot care, and are trying to work on it.
But, it needs to go further, and the madrichot need support themselves.
Is there a guidance counsellor at school, a menahelet or a social worker? Someone like that might be more approachable than the principal (have more time, more of an idea of the class dynamics, and more skill in intervening).
Are you connected with the mothers of dd's friends? Get together, compare notes, and work as a team. It helps the mom feel less alone and more in control, and that conveys a stronger message to the school, even if you end up being the only one to approach the administration. It also shows your dd that you are being proactive, that you are in her corner defending her, and that she isn't the only one feeling the tension or being targetted. You can do all this without naming names, or describing what has happened to other girls.

Finally, expect it to take time to find a resolution - not only does it take time for the administrators to get to work, it then takes time to implement strategies and for the girls to learn better behavioural patterns (both the victims in their responses, and the perpetrators in their initial behaviours).

Thank you! I'll have this discussion with her.

I spoke with the women who is in charge of 7-8 grade on the yishuv and she said she will investigate. Not counting on her too much...

I guess I was really lucky growing up. I was with a nice bunch of girls and we all got along well together no matter who was in our core group of friends.