Before you unlock jobs via the Logistics Division, you can chat up your family and drain them of money every week. The sister perk doubles the available money. At school, immediately start in on Sandra since she starts at 40 favor. Chat up other girls as available, get phone numbers at 60 favor, and don't neglect phone interactions.
With 6 charm, 90 favor with a girl, and catching her before class, you can skip class with her and take all her money for about 35 favor. Otherwise, for 2 charm and about 15 favor you can get $20. The small favor route is stronger because the significantly lower charm cost lets you also tap the girls for influence.
Once you get your grades up enough to get some logistics (or global) influence, however, the easiest and fastest way to make money is through jobs unlocked at the Logistics Division. You can only get there during school hours and you either need to fly there or get somebody to drive you. The first job you can unlock is the Janitor job in the school. Doing any job at the school currently costs 40 energy, but as long as you succeed you will make at least $50 per time slot you work. Successfully working may also give you global influence, school influence, and other benefits.
To increase the money you make from working, you can use either logistics influence or global influence to increase your pay at a particular job up to three times. You can also unlock level 2 jobs, provided you have the skills to do so, which have a slightly lower chance of success but a higher payout overall.