After we collected and integrated the views of the students from different classes on the currently limited locker number, we have found out that there are some room for improvement in our current locker policy.
Do you intend "after we collected and integrated the views of the students from different classes on the currently limited locker number" to refer to
1) a specific time in the past , which attracts the simple past tense, or
2) a period from a time in the past to the present, which attracts the present perfect tense?
The English language requires you to make your mind up on this!
(Incidentally, if 2) we would probably say in the period since rather than after).
- After we collected ... we found out.
- Since we collected ... we have found out.
- Now that we have collected ... we have found out.
As teddy says, it depends on what you want to say, or at least what time-perspective you want to emphasise.
The first is his (1): It just says that the collection happened, then the finding out happened.
The second is his (2): At some moment during the period since the collection, we found out. (It's a standard construction: since + simple past ... followed by the present perfect in the main clause.)
The third refers (in both the subordinate and main clauses) to a present state resulting from a past action, so the present perfect is used in both clauses.
You could say "After collecting and integrating ..., we ...", but a clause beginning with "after we collected and integrated" cannot modify a present tense verb such as "have" unless "after" has some sense other than "later than when".
A phase beginning with "after we collected and integrated" might, however, modify a past participle:
This room for improvement found after we collected and integrated ... is .... We have found, after we collected and integrated ..., that there is some room ....