As a high school teachers I would definitely be making contact with the school as soon as possible. Her Year Advisor (or similar) would be a good start. They should be able to help you make contact with her teachers for this year. Most schools operate with an "assessment schedule" giving a guide to what assignments are to be given and when throughout the term/semester/year - this way you know when she "should" have things to do. A diary is also a good idea - teachers don't have time to write a students homework down for them (nor should they IMO) but definitely have the ability to check a student has written it themselves - for you to see/follow up at home. Check her books daily or every couple of days - often homework is to finish what was begun in class and this will be easy to see.

I would be trying to get email contacts for her main teachers (English/Maths/Science/History/Geography etc) It is a good way to make contact without playing "phone tag" (getting teachers on the phone/teachers being able to get a "line out" can be tough) Ask her teachers to email you when something isn't done - then you can start the "punishments" - I know the kids I teach respond well when their parents take things like their mobile phone and internet priviliges away

You should also be able to ask for a progress report halfway though the term - to gauge how she is going.

All the best with it - teenagers and school can be tough