According to a report appeared in today's Daily Express more than 1.9 million students are enrolled in 17000 religious seminaries (madrassahs) of which around 70% i.e. 12000 belong to Deoband school of thought, around 20% i.e. 3500 belong to Barelvi school of thought, around 2.2 % i.e. 380 belong to Ahl-e-Hadith, 2.3% i.e. 390 belong to Shias, and 2.38% i.e. 405 belong to Jamaat-i-Islami.
This report quoting some authentic sources from federal interior ministry claims that 90% madrassahs' students arrested so far for their involvement in terrorists activities belong to a particular sect (which I believe is Deoband).
Source: Daily Express 7th April, 09