Ms Bell, who has been in charge of the information technology quango for the past 41Z2 years, announced her decision to step down at last week's British Educational Training and Technology show.
She said: "One of the great achievements of NCET has been in getting IT on the political agenda. There is a lot of good and innovative practice in UK schools but we want to get it used in every school and college."
"I have worked in the area for 15 years and there is still an awful lot to do. At the moment only about one-third of teachers in the UK use IT regularly, and that is nowhere near enough."
Ms Bell's five-year contract was due to terminate in July. She will now be taking on research and consultancy work after NCET council members failed to persuade her to reapply for the job. Mike Littlewood, NCET's finance director, will take over temporary charge of the organisation until a new chief executive is appointed.
Heather Du Quesnay, chair of NCET, said the education service owed a great deal to Margaret Bell. "She has made a major contribution to establishing the UK as one of the leading nations in the use of information technology in schools and colleges. We shall miss her energy and commitment."