Yeah I would do it through your tights, but if you feel embarrassed you probably are better off nipping to the bathroom. How old are you can I ask? I think you should embrace it, honestly tiny children are the worst for saying insensitive things and asking weird weird questions, but after maybe 2 days of seeing me do it they don't even take a second glance. Hiding away to do a blood test must be a real pain, it's good for others to see what you have to deal with and manage on top of all the regular stuff and less of a pain for you! You probably draw more attention to it by disappearing throughout class. I did my tests etc quite visibly at school and nobody was very interested