If we define "tone" in cinema as the attitude towards the subject of a particular film, then I think that the tone of To Sir, With Love is a resounding love for the promises and possibilities of education. The attitude that emerges from the film speaks to what education can accomplish. It speaks to how children can change and how educators can be the agents of such change. The effect this has on the audience is to construct an admiration for teaching. Few films are able to be so direct in ensuring that the audience leaves with a respect for educators.
At the same time, the film speaks to how powerful a force educators can be in the lives of children. Mr. Thackeray connects with his students in a personal and meaningful way. The film's tone leaves the audience to shudder as to what might have happened to the children had Thackeray not entered their lives. The attitude that the film takes towards Thackeray and teaching, in general, is one in which the audience is left to reflect about teachers in their own lives and the essential social function that education of youth serves.