Schools half term is normally mid February - this year it's w/c 12th Feb for nearly all schools. Easy to spot - it's when the prices are sky high compared to the rest of the season
Fortunately for us - my kid's half term is the week before so we're going 6th Feb, just missing the rest of the world's children and the shocking increase in prices..
you can either book through the uk companies, or book through a very friendly Bulgarian comany (this one) Maria the rep in Pamporovo, looked after us brilliantly last year, and am sure will do the same this year!