Can I use "Teachers portal" instead of "teachers' portal"? Which of them is more accurate?
The problems with "teachers portal" is that it is a noun1 + noun2 combination with noun1 acting adjectivally.
Noun1 is thus only very rarely inflected - and this includes the plural 's'. This gives the impression of noun1 being uncountable.
Thus "teachers/teachers' portal" should be "teacher portal".
For example, the "Department of Languages" would be "The Language Department." and then there is "The student's room" - a room for one student; "The students' room" - the room for many students, and "the student room" - the room associated with students in general.
As an aside, the genitive 's' is only loosely associated with possession, it is more associated with some sort of connection or attribute. It is this that licences the use of the genitive 's' with inanimate objects that cannot, of themselves "possess" anything.