Are there common idioms to mean "becoming a teacher/pastor/'insert profession here' ", used in a figurative way?
In some circles, especially religious ones, such occupations are considered vocations (from the Latin vocationem, meaning a calling).
Choosing such an occupation is sometimes described as answering the call. For example, in Lutheran schools, teachers who view their path as responding to a divine plan are referred to as called teachers.
I can think of no universal expressions for taking up a vocation.
There are a handful of set phrases, e.g. someone who becomes a sailor has gone to sea and someone who becomes a priest (usu. Catholic or Anglican) has taken the cloth, as Edwin Ashworth notes. I imagine that there may be jargon within professions which may not be widely known.
If you want to avoid ordinary expressions of becoming or joining, you could employ metonymy, combining don or take or some other word for acquisition with clothing or tools or other items associated with a particular role or identity:
The new academy graduate donned the badge of the Lancaster County Sheriff's Office.
His business in the city a failure, he returned to the family homestead to pick up the hoe once more.
She was hired to teach calculus, but with the softball team in dire straits, she offered to take up the whistle as well.
But note that none of these examples are set phrases, and that they are dependent on the symbolism being identifiable by the audience. If the police in your country do not carry badges, or if the farmers are all poultry farmers, then the meaning is lost. There are so many lawyers and consultants in Washington, D.C., to the apparent exclusion of all other occupations, that I have joked with my friends that new residents should be asked "suit or polo shirt?" But probably no one outside Washington would understand the reference.