Hi Jillian,
Great question. The most international schools are found in the capital, San Jose, and its suburbs like Escazu and Heredia. Tuition is about $500 – $1,000 a month and up for a quality accredited school (often accredited in the U.S. so that credits transfer easily). But these days there are also good schools all around the country, including beach communities like Jaco, Tamarindo, Flamingo, and Playas del Coco, and they’re usually cheaper than in San Jose. Some are bilingual, some English only. You can find a list here of the some of the top schools (scroll to the bottom of the page):
https://internationalliving.com/country_clubs/costa-rica/moving-to-costa-rica/
Also, worth a mention are two schools in the mountain community of Monteverde.
The Friends School: http://mfschool.org
Cloud Forest School: http://cloudforestschool.org
There are also many many small, independent bilingual schools throughout the country. Very reasonable tuition. But the level of education isn’t as high as the larger ones, in my opinion.