Translation:The teacher prepares a new method for learning languages.
Can we say "La instruisto PRETAS novan metodon por lerni lingvojn." (and is there a slight difference?)
No, it would mean "The teacher is ready a new method for learning languages." So the sentence would be ungrammatical.
Mihxal is right--you can't say that, because preti means 'to be ready'. But you could say "La instruisto pretigas novan metodon por lerni lingvojn", with pretigi formed using the suffix -ig 'cause'.