Is it permissible for a teacher to use a portion of the children's breakfast money for herself and other teachers, knowing that she provides breakfast that is better and cleaner than what is sold?

One must differentiate between the action of a seller and that of an agent. The former usually profits, while the latter only profits with the knowledge of the principal. Collecting money from children to buy food is considered an agency, and it is not permissible to take anything from it or from what was purchased with it, except with the knowledge of their guardians. This is because they are legally restricted due to their young age and limited understanding, as stated in the Almighty's saying: {And test the orphans until they reach the age of marriage. Then if you perceive in them sound judgment, release to them their property}.