"Teacher" and "professor" are used rather differently in Greek and in English.
"Teacher" in Eng. is used from elementary through high school. "Professor" is for college/university.
"Καθηγητής" (teacher or professor) is used in Greece in middle school, high schools and college/university.
Since here there is no indication of the exact level of the "institute" nor the title of the "καθηγητες" we can also use "teacher".
A university in English may have the name "institute" (e.g. MIT) so would have "professors/ καθηγητές. But you could also have a "Beauty institute (ινστιτούτο ομορφιάς) which would then have "teacher".
So, with all that which I hope clarifies the situation we will add "teacher" to the vocabulary list.
The usage of professor differs markedly between US and British English. The the US, all university teachers are called professors, but in the UK that title is reserved for a very small number of very senior academics who are given leadership responsibility. Which of these senses does καθηγητής encompass?