Okay, I have this feeling that the phrase 'fill up the form' sounds a bit weird, so I googled it and a lot of websites told me that people usually say 'fill in' or 'fill out' when talking about forms and that 'fill up' was wrong. But I found a couple of (reliable) webpages where 'fill up' is used in this sense (I can't give you the links because apparently I don't have enough posts).
My question is essentially- is 'fill up the form' considered bad English? Would it be fair for it to be marked wrong on a grammar test? Is it used anywhere by native speakers of English?
Not a teacher.
Forms are filled in or filled out. Not up.