Can u do a fourth alevel externally while being enrolled in school??????

So basically i missed my requirements for biology a-level by half a grade to do it as an a-level in my sixth form and they definitely wouldn't let me do it, i ended up choosing law ,which i hate, psychology, and sociology but i only decided half way through year 11 i really wanted to do medicine, so would i be able to enrol for biology externally while still doing a-levels in school?????

What about practicals, besides for Medicine they usually require you to do at least both Chemistry and Biology.

You will need two sciences for Med - and Chemistry is the most important.