Highschool probably primarily, since they're supposed to be more uptight there. In college you might get an old stickler or an edgelord teacher who cares about it.
Less accepted in formal writing in an academic setting. Thought that was the obvious subtext here?
I think it depends on the teacher. I work with mainly college professors (which have been forced into a quasi-highschool role because highschools pass students even if what they write isn't comprehensible), and some of them will go easy on you as long as they can follow what you're saying and you make a good argument. Some will ride you hard for every 'til' and misplaced 'your' or 'they', and some will start marking them down if you pull shit like fail to cite sources or show zero effort in revising your papers. It just depends on the teacher and the mood. In my state they're even more pissed off because a measure was passed to force all incoming college students into college level composition regardless of what level they're actually at. They're getting rid of all the remedial courses that would help students catch up.
If you're legitimately curious, there are most likely formal grammar handbooks out there which cover it. I don't bother with them, mostly out of spite for one of my bosses who lives by them. I think also most of them become too rapidly outdated given the pace of change in formal English writing. But in 2010? I could see til/till being marked down.