Friend that one had been to school with

　Yesterday, I met my friend from university. He came my university as a recruiter of a company and introduced his company to students. And today, I ate lunch with him, then later we talked about various things each other. It was so fun to talk with a classmate after a long time. This is why almost all my classmates got into companies when we graduated our university, and there is no classmate in my research building. I sometimes feel lonely, however, I also sometime feel as if I were a boss of students since all students in my research building are younger than me. He-he.

• Friend that one had been to school with
• Friend that one had been to school with

• Yesterday, I met my friend from university.
• This sentence is perfect! No correction needed!

• He came my university as a recruiter of a company and introduced his company to students.
• He came to/visited my university as a recruiter of a company and introduced his company to students.

• And today, I ate lunch with him, then later we talked about various things each other.
• And today, I ate lunch with him, then later we talked about various things with each other.

  'to talk about something with someone' :)

• It was so fun to talk with a classmate after a long time.
• It was so fun to talk with a classmate after such a long time.

• This is why almost all my classmates got into companies when we graduated our university, and there is no classmate in my research building.
• This is why almost all my classmates got into companies when we graduated from our university, and there is are no classmates in my research building.

  we usually just say 'to graduate from university/college' -I changed 'is' to 'are' because I think the plural sounds better in this sentence :)

• I sometimes feel lonely, however, I also sometime feel as if I were a boss of students since all students in my research building are younger than me.
• I sometimes feel lonely, however, I also sometimes feel as if I'm were the boss of the students since all the students in my research building are younger than me.

  The sentence structure 'as if I were...' is correct but in this case I chose to use the present tense 'I am' since it sounds more natural :)

• He-he.
• This sentence is perfect! No correction needed!

It's always nice to catch up with a friend after graduating! :) Sounds like you had fun

• This is why almost all my classmates got into companies when we graduated our university, and there is no classmate in my research building.
• Because almost all (of) my classmates got into companies when we graduated from our university, and there are no classmates in my research building.

  I think maybe you made a mistake using "this is why" in your original sentence, so I corrected it to something more logical in the context of your paragraph. Hope this is what you wanted to say!