Previously, many foreign nationals used to come to the UK to "moody" colleges under student entry and then just continue "studying" at the college while actually working. Some of the colleges were little more than a PO Box number.
The Points Based System (of which there are a number of different categories) with licences for colleges/schools was introduced for UKBA to have better control over this.
BG and Romanian nationals can not work in the UK without a visa.
Students are allowed to work a few hours per week during term time and a bit longer (can't remember the exact figures even though I have just read them
) during the holidays.
Bulgaria and Romania are still considered to be Accession countries which should have ended at the end of this year due to the 5 year Accession period being over. But, yesterday it was announced that the restrictions have been extended until the end of 2013.
http://www.ukba.homeoffice.gov.uk/sitec ... strictions