Supporting children after they turn 18 is a big issue and should be thought of in advance when you are settling on your final order of child support.
There is no “child support” after the age of 18 or graduation from high school because there is no “child”. Support after the age of 18 is called “post secondary support” and can be resolved in a number of different ways. WA state has a separate and distinct section for this in our mandatory forms, and it is required to be addressed. One way is to agree to decide it later and if you choose option, one party has to initiate proceedings BEFORE the child turns 18. The other options are whatever you can agree on. If you cannot come with an agreed plan, the default is to go to court later.
There are different ways to pay for college, different ratios an be assigned to the parties and sometimes to the child depending on the economic circumstances of the parties. A very popular formula is that the parties agree to share the costs of room/board and tuition proportionally to income after any college savings and/ or scholarships have been applied. Usually this is capped at instate tuition at the University of WA. Sometimes the major income earner agrees to cover all costs. Sometimes parties want the child to be responsible for a portion.
Usually the parties pay the institution directly as it is a large sum of money to cover for the other party.
If your child is nearing the age of 18 and post secondary support has not been decided, do see an attorney to help you as it might require court involvement and either a hearing or mediation depending on your dispute resolution clause in your final orders.