SubhanAllah x 100
Alhamdulillah x 100
AllahuAkbar x 100
La Ilaaha Illallaah x 100
La Hawla Wa La Quwwata... x 100
Durood x 100
Astaghfirullah x 100
“subhanallahi wabihamdihi subhanallahil azeem” x 100

Better it is to be in the state of wudhu and do zikr facing the qiblah and do it calmly - don't hurry, recite the words properly.

Better it is to do it post fajr and asr but if you can't find time [school/college] do it at other times, fix a time for this.