Issue them a credit card. Before you shriek, credit cards for high school students can teach the young adult about good and bad credit at an early age. A parent or guardian will have to co-sign with the student in order for the credit card to be issued. This will help build their credit reputation early on and also teach them a lot about debt management. It has been shown that students issued with a credit card have better developed sense of responsibility.
Co-signers as well as the students should look at all the available options first, before choosing a bank. Some banks and companies will try to charge unreasonable rates due to the student not having any credit record yet. With the amount of credit card companies to choose from, it is a good idea to do your research thoroughly. Some cards will have more reasonable rates than others, while some will come with high APR and a high annual fee.
Due to inexperience, students should always ask for help from a parent or guardian. By acting as co-signers they will help choose the best deals. The co-signers will at the end of the day be responsible for payments if the student is unable to make them.
If you are worried about them running up the bills, issue them a prepaid credit card. This will teach them about financial management without any risk. The amount of credit you load onto the card will be their spending limit.
Instruct them how to use their card when the card is issued. Tell them that the card should only be used for emergencies and not for everyday spending. Reward them if they manage to pay their bill by themselves, without running into debt. This will help to boost their credit record too.