My experience with sertraline is that it does get worse before it gets better. The medicine needs time to build up in the system and sometimes dosage tweaks are needed. Sertraline also carries a risk of an increase in suicidal thoughts with teenagers so keep a close eye on her, just incase.
Whilst the medicine is building in her system, the best thing to do is to focus on getting her eating well, getting some gentle exercise each day, getting her into a healthy sleeping routine and socialising as much as you can. I know first hand that these things feel impossible when in the grip of anxiety/depression, but encourage her as much as you can and keep reminding her that how she is feeling just now is temporary, not permanent. Point out any achievements, no matter if it's just eating a biscuit, she won't be able to see when she is doing positive things.
It might help to get David D Browns book Feeling Good and reading it with her, help her with the many exercises it contains. In his book, he says that most depression is self limiting, which I found comforting even if the jury is still out on that as a whole.
One of the biggest issues with depression/anxiety is ruminating, dwelling on your thoughts. Try to keep her gently busy, be it a little bit of baking or a craft activity, or just a little walk.
Look after yourself too. It's so easy to get lost in trying to help and care for someone with mental health troubles, especially when it's your child. You need to make sure you take good care of yourself so you can care for her.
What with covid, climate change, the Russian invasion, it's hard for adults to cope, let alone our children. Do your best to help her see the good in the world too.
With sertraline, in general from the many people online and offline I know who take sertraline, by around two weeks some begin to have a better period of time in the day and this gradually increases over the weeks. It's totally normal to have a good day and a stretch of bad days in the beginning and unfortunately for us women, AF can cause a dip each month too.