OP wants the computer for school, he might take a programming class, might want to play with Xcode and to install that it will take at least 10GB, he might like music and that at least will use 15GB for a small collection of 1000 songs (mp3) recorded at 256kbps to 320kbps. OP might want to download some videos, ebooks, photos... 120GB will be gone and by empirical experience I can tell that the less space you have available in your SDD drive the slower it gest and the machine starts to choke... Been using computers for 35 years.
I forgot to mention installing windows in a virtual machine, that will take a lot of space... Specially I f your are a student some times you need windows.