How to write this sentence correctly?

[rukmini]

The teacher had to read the assignment again for the students,since they had difficulty understanding what to do after he had explained it the first time.

[Allen165]

NOT A TEACHER.

I don't think "read" makes sense; perhaps you meant to write "read out."

[Gillnetter]

The teacher had to read the assignment again for the students,since they had difficulty understanding what to do after he had explained it the first time.

"Read the assignment again" is fine if that is what the teacher did - the teacher stood in front of the students and read the assignment to the students.

Another term that could be used is "explain" - the teacher told, in detail, what the assignment was.

You also use, "go over" - the teacher had to go over (explain again) the assignment again - the teacher spoke to the students and told them what they had to do.

[Allen165]

"Read the assignment again" is fine if that is what the teacher did - the teacher stood in front of the students and read the assignment to the students.

Another term that could be used is "explain" - the teacher told, in detail, what the assignment was.

You also use, "go over" - the teacher had to go over (explain again) the assignment again - the teacher spoke to the students and told them what they had to do.

I think if you want to keep "read," you have to make minor changes:

"The teacher had to read the assignment to the students again because they...."

[Gillnetter]

I think if you want to keep "read," you have to make minor changes:

"The teacher had to read the assignment to the students again because they...."

It could be either way - if the teacher read out loud and directed the reading in the direction of the students, then "to" is correct.
If the teacher read the assignment, and the reason was to inform the students, then "for" is correct.
If you read something for a person who cannot read English, you may be reading to the person but you are also reading for the person.