How do you say ABANDERADO in English?

Let's begin by stating there is no thing such as ABANDERADO in the educational systems of any of the English-speaking countries.

A good student is a good student, he or she has the best scores and receives honors accordingly, but as part of that recognition there is no such thing as giving him or her the honor of becoming a STANDARD- BEARER. And that expression is the answer to how we say ABANDERADO in English, though the word standard itself does not necessarily mean bandera since also other emblems (even a torch) can be part of the definition of that word. Then in the specific case being mentioned here, that is the greatest honor granted to the best senior student in a high school, the translation can also be FLAG-BEARER.

It is not that there are no ABANDERADOS in the English speaking countries, but they are not students. This honor is usally conferred by military institutions to an outstanding cadet or soldier. Standard-bearers are usually seen in parades or similar events during national holidays of course.
In Ecuadorian high schools the best senior student  bears our national flag, the second best and the third best bear the city's flag and the other the school's flag respectively. Each FLAG-BEARER is accompanied by two escorts (ESCOLTAS in Spanish) who are chosen according to the scores and other parameters as well. Together they are the best and brightest we can offer. In the pics you can see the flag-bearer of Colegio Politécnico (where I work) and  some of the others. From left to right: Fátima Hurtado, Doménica Terán, María del Cisne Hernández and the crying  girl is Emily Pinzón.