A few possibilities:

• Not very formal but still polite

  Bonjour,
  Bonjour à tous,

• More formal

  Bonjour Madame, bonjour Messieurs

• Very formal (Laure’s suggestion)

  Madame, Messieurs

  or

  Madame le professeur, Messieurs les professeurs

Personally, for teachers that I know, I would pick the first, “Bonjour,”.
I would add “Madame” or “Monsieur” with or without the family name or the title only if there is one person (or two of the same gender).
Otherwise, it is a bit long and too formal.

E.g.:

Bonjour Messieurs les professeurs,
Bonjour Mesdames,
Bonjour Monsieur Martin,
Bonjour madame la professeur,

But, if I don’t know the person I’m writing to, I would use the very formal form.