Challenges student visa to tourist visa

Hello. I would like to ask how could I change student to a tourist visa and what challenges I might encounter for it?
Finished my language school this week and was told I have to leave the country within the next one month ( have valid student visa till next year Nov ). Although I have received an offer as a head volunteer for one of the conferences that are going to be held in February and I need to remain in Tokyo to support them.
When changing the visa status will they ask for reasons and is it a valid one to remain here? what other challenges might pop up while going through this process?
Thank you sincerely in advance for your answers.

What is your nationality? I think it matters as it determines the length and process for getting a tourist visa.

Finished my language school this week and was told I have to leave the country within the next one month

Who told you this? If it's anyon other than an immigration official, they are probably wrong. In principle, you can stat for at least 3 months after leaving school. You could be an exception, however, so if you want to be sure, go to your local immigration office and ask.

If you want to change your status from "Student" to "Temporary Visitor", see this: http://www.immi-moj.go.jp/english/tetuduki/kanri/shyorui/02.html However even if the change is granted it is not guaranteed that you will get 3 months.

just book your returning ticket in next 2months or 3months go the immigration and explain the reason why you want stay more in japan and with your ticket reservation they will give you according to your ticket itinerary once is not more than 3months.