Robert Boulos, of Unity High, said: "It's a very fair verdict, she could have had six months and lashes and a fine, and she only got 15 days and deportation," adding they would not appeal the decision.
Gibbons is expected to serve out her sentence in the Omdurman women's prison near Khartoum before being deported.
A British foreign office spokeswoman said: "We are extremely disappointed with the sentence and foreign secretary David Miliband has summoned the Sudanese ambassador to explain what has happened."
Muhammad is a popular name in Sudan but Islam forbids any physical representation of the Prophet Muhammad.