If you are successful in school with time to spare...

If you are successful in school with time to spare, and you want to get ahead by doing extra classes online, FLVS will work fine, and will ultimately be beneficial.
If your school is forcing you to do this regardless of your scholarly status, then you will probably have a bad time.
While the ability to take an extra class over a long period of time, doing a few assignments a week, can be a very good idea, if you are struggling with classes as it is, then you may have trouble. See, the "Go at your own pace" thing is quite fake. Teachers generally have a minimum of one-or-two assignments a week, which would be much less of a problem if said assignments weren't structured like projects; all of them are very time consuming.
If you can spare a good amount of time every weekend, then FLVS could definitely work in your favor. However, if you have enough work already, then consider entering FLVS with caution or avoid it all together. (I say enter with caution, as many public schools are making an online-school course required)