How to sell services to other cities in SimCity 5?

Is it possible to sell education/ambulance/fire/police services to other cities as well as water or electricity?

For example, people from other cities are already coming to my schools. How to make them pay for this?

The short answer is: this no longer matters because the servers for this game are gone. Only offline play is available, which means only cities you control in the region on your computer can interact.

The longer answer is that cities within a region interact asynchronously as described here. This allows each city to continue to function as though the other cities were being played even when they aren't. Since time passes differently for each city, resources can easily be created/wasted.

Available surplus resources (power, water) are capacity split as described here. The consumer of surplus resources pays a modest amount per day if they "purchase from region". This is much cheaper than generating the resources locally. The producer of surplus resources gets paid a meager amount which can be observed in the budget screen. The amount is automatic. Note: due to the asynchronous nature of game play, it doesn't matter how many days the producer supplies the resource - nor how many days the consumer gets the resource... the surplus resource is stably provided each day.

Assigned service vehicles behave as described here. The assigning city may get paid a meager amount for each vehicle - if so, it will appear in the budget screen. If so, this would be well below the maintenance cost of the buildings to operate those vehicles. The assigned city pays nothing. Assigned police cars will bring back criminals, assigned garbage trucks will bring back garbage, assigned ambulances will bring back sick and injured people. Note: this is basically a negative for the assigning city. Generally, you should only assign vehicles when the problem they would otherwise solve would spill into your city (crime). Omega trucks are a special case.

Lastly, schools. If students come from other cities to your community college or university - the benefit to your city is local tech points. If students come from other cities to your high school or elementary school - there is no benefit to your city.

If your students leave the city to attend school out in the region, then your city will get the residential education those students earned. Caveat #1 this is no different than attending a local school. Caveat #2 students leaving the region tend to glitch out and never return. If you find yourself with fewer local students than you should, remove all your school bus stops (to stop/limit regional education) and bulldoze/cycle your residential buildings to get them to move back in.

As such, regional students are to be avoided by providing enough education in each city that no one decides to leave.