Till and Until are synonyms. They are both prepositions of time and refer to a period of time leading up to a specific time.
I worked for the bank from 1989 until 1994.
We were happily married till I discovered she was having an affair.
In both these examples we are talking about a period of time which started in the past and carried on until a specific time in the past. The end time is introduced by until or till.
Till was originally used more in the north of England while the south preferred until. Perhaps for this reason until is regarded as slightly more formal and is more common in written English than till.
Having said this, both are pretty much acceptable and interchangeable. Your students can use until any time; explain to them that till is slightly more informal and whilst they can certainly say it, it’s perhaps best used only in informal writing.
In addition, you’ll occasionally see ‘til in phrases like:
Shop ‘til you drop.
Here the original word is until and the apostrophe shows that several letters have been removed. It is wrong to write it without the apostrophe as there is no preposition, til, in English.
There are a number of theories regarding the derivation of these two words. The most common belief – which is wrong – is that till is a shortened form of until. In actual fact the two words are completely separate and have never been joined.
Until is a form of unto meaning as far as or up to. Here, un comes from the Old Norse und (which is itself related to the Old English end).
Till comes from Middle English and originated in various Scandinavian languages. It may be derived from the Icelandic tili which means aim or purpose/goal, itself related to Old High German sil meaning aim or mark.