集まる and 集める: meeting places and collected objects
Distinguish where people gather from materials someone collects, and compare collected objects, sources, and results.
Specify a meeting place
明日は九時に駅前に集まってください。 asks people to gather in front of the station at nine tomorrow. With 集まる, read あつまる, people gather; the two に phrases mark time and place.
全員が集まったら、出発します。 means that the group will leave once everyone has gathered. 全員 distinguishes a full gathering from only some people arriving.
Gather people or materials
係の人が参加者を入口に集めました。 says that the organizer gathered the participants at the entrance. 集める, read あつめる, identifies who brought whom together.
学生からレポートを集めました。 means that someone collected reports from students. レポートを marks the collected material, while 学生から identifies its source.
A hobby and the required quantity
旅行先の切手を集めています。 can describe a hobby or ongoing activity of collecting stamps from travel destinations. It need not mean picking up stamps at this exact moment.
必要な資料がまだ集まっていません。 says that the required materials have not yet all been gathered. Starting to collect them does not establish that the required result has been reached.
In 学生からレポートを集める, are the students or the reports being collected?
Show the answer and why
The reports. レポート with を is the object, while 学生から identifies the people from whom the reports are collected.