It's worth remembering that in those days, schools programmes were, with the exceoption of regional opt outs, broadcast in most cases twice a week. AFAIR on a friday during the ITV Schools on 4 era, all programmes, except for regional opt outs, had already been shown during the week. It's quite possible that TTTV may have been broadcasting a repeat, wheras other regions were getting something else.
However, By the new academic year in autumn 1981, such regional opt outs for the English regions were abolished. By that time, only STV, Grampian, Ulster, and HTV Wales opted out.
The Border TV interval caption was a pretty psychedelic affair. The rectangular logo was shrunk a bit, with a massive INTERVAL caption underneath it. The background was a constantly changing coloured loop going from yellow to blue with about ten different colours over 1 minute - pretty hypnotic as a kid....
On the subject of Interval slides, I remember the late 70s one from Tyne Tees showed a montage of TTTV logos in various colours with the word 'interval' in very small writing in the middle of them. This had a blue line at the bottom and the Tyne Tees logo in a circle, as was common with most of their line up/programme slides. This was replaced in 1980 with a full screen design showing some abstract petals/flowers and the word 'interval' in cursive script. Both of the above were sometimes shown during the extended 10.30 interval.
My favourite interval slide was introduced around 1982 and featured a girl holding a candle with 'interval' written in large type underneath - this was a terracota/orange colour. Such slides were also seen on Sunday mornings and at the end of ITV schools when there was a few minutes to spare before the 12.00 children's programmes.
Tyne Tees also had at least two generic 'Programmes for Schools' slides, and a slide showing 'ITV Schools' written on a blackboard.
Robert, regarding the 'opting-out' discussion - there were still a few English regional variations after 1981 - I remember Granada opted out of the networked 'Documentary Re Run' from Yorkshire to show their own 'Experiment' programme in the slot. TSW, I believe, also showed a Cornish language programme in the mid-80s, replacing a Friday repeat of 'Middle English' in Summer 1986.
Whilst I appreciate that Tyne Tees may have played their own music and/or shown their own slides if opting away from the networked programme, such 'opt-out intervals' went on until 1985, and the programmes in question did not differ from the networked feed from Birmingham. On one occasion in Autumn 1982, Tyne Tees provided their own intervals for almost an entire week, despite there being networked links at all times. It does lead me to speculate whether they chose to do this to test equipment, train staff, or just to relieve a bit of mid-morning boredom! Oh, where is Tony when you need him!