Per the IRS rules in IRS Publication 970, each school has their own definition of a full time student. So you'd have to ask the school. Additionally, even though it's high school, if it's not "accredited" by the U.S. Dept of Education, then you have nothing to claim.
Generally, to be considered a full time student you have to be taking on "more than half" of whatever the school decides is a "full time workload".
Also note that as of 2018, out of pocket education expenses are not longer deductible. At best, you might get a few credits, such as the AOTC - but I don't think you can for high school. But if you plan on attending college it might not be a good idea to essentially "waste" those credits for high school.
