Beginning a sentence with a gerund?

My teacher recently marked on my paper not to use a gerund to start a sentence. I have been told by teachers in the past to use that format to vary sentence structure. It seems to make the paper flow better oftentimes. An example:

Learning to confront his problems is an important part of [character's] growth and maturity in the novel.

Is there a grammar rule about this? I haven't ever heard/seen one before.

Edit: It would also be helpful to know of any good examples or arguments I would be able to use when discussing this.

Believing your teacher would be a mistake.  (Did you see what I just did?)

I completely agree that there is nothing grammatically wrong with starting a sentence with a gerund phrase; however, I do tell my AP students to avoid the construction because it almost always leads to the use of a linking verb, as is the case with the example. If we all agree that the use of action verbs is preferable, then the use of the gerund phrase as the subject of a sentence should be avoided. I often have students write (in a literary analysis), "By using the ..." I simply tell them to cross out the 'by' and use the participial phrase instead.

Gerund as subject: Smoking is bad for your health.

See link http://www.grammaring.com/the-gerund-as-subject