Let $x$ denote the number of contestants in line before Andrej. Then there were also $x$ contestants behind Andrej and there were $2x+1$ contestants altogether. Hence, the total number of contestants was odd. Since Andrej was standing in line before Blaž, who was 19th, there were at most 17 contestants in line before Andrej and $x \le 17$. This gives us the maximum of $2x+1 \le 2 \cdot 17+1 = 35$ contestants. Since Žan was 28th in line, there were at least 28 contestants. As each of the schools has sent 3 contestants the total number of contestants must be divisible by 3. The only two numbers between 28 and 35 that are divisible by 3 are 30 and 33. Of these only 33 is odd. There were 33 contestants, hence there are 11 schools in the region.