The thing I have tried the hardest at during my school days (for resume)

I have planned a university festival tour for about 100 of my high school juniors and made it succeed every year.
Most university students feel the gap between the ideal school life and the actual one after entering their university because of the lack of information. I am one of them. I do not want my juniors to experience the same thing, so I decided to arrange a university festival where they could get many chances to communicate with the university students.
The event I put the most effort into is discussion meetings made up of 3 to 5 people including the university students. I asked 30 friends to cooperate for the event and made it possible so that my juniors could easily ask them many questions.
As a result, they really liked it and told me ‘I could get all of the information that I wanted to know!’ The participants increased threefold in the past 2 years.
I took great pleasure in meeting people’s demands  from this experience of being a volunteer organiser.

Ryo this is really great English. The corrections I added are only very minor. In English organise has an s in American English is would have a z.

My only slight confusion was your use of the word juniors. In the USA "juniors" in High School would be the students in the year before last, but I think from the sense of your piece you are referring to any high school students younger than you and it is correct that juniors can be used in this sense but we may just say "high school students"

In England "juniors" generally is the school for children aged 7 to 11 (part of primary school). Just to add to the confusion of the Englsih language!