as-salam greeting (saying assalamu alaikum) is one of the great sunna’s (prophetic manners) but most of the teachers do not apply this sunna and most of them use other types of greetings instead. it is not a problem if you say good morning or good evening, but such kind of greetings should not be said before as-salam. therefore, saying any kind of greetings without making as-salam firstly is not valid. moreover, some teachers instruct their students to stand up for them when they enter the classrooms on the ground that such standing is a sign for good manners and respect toward their teachers; however, such standing cannot be regarded as a sign of respect because it goes against the islamic teachings. 

1- anas bin malek, may allah be pleased with him, said: (although the companions were of high respect and love to the prophet, peace be upon him, they never stood up when he enters because they know that such action is abominable).

2- the prophet, peace be upon him, said: “he who likes people to stand up for him, let him know that his seating will be in the hell".

saying as-salam greeting whenever you meet or leave people is great source for increasing your merits and good deeds; therefore, the prophet, peace be upon him, instructed his companions and urged them to keep doing this great sunna.

3- the prophet, peace be upon him, said: “if you meet your (muslim)brother, greet him (by as-salam greeting), and if a tree, wall or stone separated you (while walking together), greet him when you meet him again”.

extending as-salam greeting to others helps in spreading love and respect between individuals and groups; therefore, it is more recommended to use this greeting by the teacher upon entering to and departing from his classrooms.

4- the prophet, peace be upon him, said: “you will not enter paradise until you believe, and your belief will not be perfect until you love each other; so, shall i tell you about something which makes you love each other? extend as-salam greeting among yourselves”.

if the feelings of respect and love were exchanged between the teacher and his students, the students will be more ready to accept their teacher and learn from him because every one of us is more ready to accept anything that he loves.

another important matter is that the teacher should say as-salam greeting upon departing the classroom.

5- the prophet, peace be upon him, said: “when any of you reaches his seat (in any gathering, meeting or place), let him firstly extend as-salam greeting to others. then if he likes to sit down he can do so, and when he decides to leave, let him say again as-salam greeting, because extending as-salam greeting is of the same importance upon coming in and going out".

summary:

1) it is recommended to extend as-salam greetings to the students upon entering the classroom.

2) extending as-salam greeting is a reason for gaining the forgiveness of allah and increasing the good deeds.

3) extending as-salam greeting is a reason for spreading love and respect between the teacher and his students.

4) it is recommended extend as-salam greeting upon entering to and departing from the classroom.