because the teacher is the subject. It should have some commas "The teacher, along with her students, is viewing a film."

Like so: "The teacher (along with her students) is viewing a film." "Along with her students, the teacher is viewing a film." "The teacher is viewing a film, along with her students." So in short, it is the teacher who is viewing the film. The students are just extra information.

we use "is" because the subject in this sentence is the word "teacher" not "students". since teacher represents one person...we can't use "are" to describe her actions, we will use the singular form...which is.."is". hope it helps! =)

Because we're only talking about the teacher. Her students might be not viewing the film right the moment. Like "the man beside the girl is smiling".