If you are a U.S. citizen receiving credits while in the Czech Republic for 90 days or longer, you must apply for a student visa. If you are not a U.S. citizen, you should obtain information from the Czech Embassy or a Czech Consulate regarding the immigration requirements for citizens of your country.
Czech Government & Visa Resources
Guidance for Obtaining Student Visa to Study in the Czech Republic
Embassy of Czech Republic, Washington, D.C.
Consulate General of Czech Republic in Los Angeles
Colorado residents are within jurisdiction of LA consulate.
Czech Travel Resources
Guide for Studying in Czech Republic