Bilingual isn't "with an English class"
A school with one hour of English a day teaches the language as a subject. A bilingual school uses it as a language of instruction: there are full subjects in English —reading, composition, math, debate— and students think in the language instead of just translating it.
Always ask how many hours per week are taught in English in the grade you're looking at, and which subjects those are. At Colegio Real de Minas, for example, Middle School adds up to 13 hours a week and High School 10, with a business focus.
The program matters as much as the hours
Hours without a method are wasted. Ask which program the school uses, whether it's designed in stages and whether it has a measurable goal. AMCO, for instance, has At the Top for the early years and Speed Up for Middle and High School, aimed at CEFR levels B2–C1.
Certification: proof of the result
A good bilingual school doesn't ask for faith: it certifies. Ask in which grade English is certified, with which institution and whether the certification is internationally recognized.
Accreditation and continuity
Check that the school is accredited by the State Education System and, for High School, which official curriculum it follows. Think about continuity too: switching bilingual models between Elementary and Middle School usually costs a year of adjustment.
