Learn Portuguese (Portugal): Bem-vindo, professor!

Bem-vindo means "welcome," and it changes depending on who you are talking to! Because we are talking to a male teacher (professor), we use the masculine singular form bem-vindo. If the teacher were female (professora), we would say Bem-vinda, professora!

How would this change if we were welcoming a group of teachers?

You would need to make both words plural! If it is a group of male teachers (or a mixed group of males and females), you would say Bem-vindos, professores! For a group of only female teachers, it would be Bem-vindas, professoras!

In English, a "professor" is usually only at a university. Is it the same in Portuguese?

No, in Portuguese, professor (male) or professora (female) is used for a teacher at any level of education, from primary school all the way up to university.

Don't we usually put o (the) before titles like professor?

Usually, yes! When talking about a teacher, you would use the article and say o professor (the teacher). However, when you are speaking directly to the person to greet them (like in this sentence), you drop the o.