Complex sentence: A storm of outrage swept through the school, apparently spearheaded by teachers who resented that Sullivan and Keller had been granted use of the facilities although not employed by nor officially registered with the school.
Split into simpler sentences: A storm of outrage swept through the school, apparently headed by the teachers. Some suggest that they were also incensed by the fact that Sullivan and Keller used the facilities although they were neither employed by nor officially registered with the school.