QUICK ANSWER
"Secundaria" is a form of "secundaria", a noun which is often translated as "high school". "Colegio" is a noun which is often translated as "school". Learn more about the difference between "secundaria" and "colegio" below.
la secundaria(
seh
-
koon
-
dah
-
ryah
)A feminine noun is almost always used with feminine articles and adjectives (e.g., la mujer bonita, la luna llena).
1. (education)
a. high school (grades 9 to 12) (United States)
Regionalism used in the United States
La señora Díaz fue mi profesora favorita en la secundaria.Mrs. Diaz was my favorite teacher in high school.
b. secondary school (United Kingdom)
Regionalism used in the United Kingdom
La biología fue mi clase favorita en la secundaria.Biology was my favourite course in secondary school.
c. middle school (grades 6 to 8) (United States)
Regionalism used in the United States
Cuando Benito cumpla 11 años, irá a la escuela secundaria.When Benito is 11, he'll attend middle school.
d. junior high (grades 6 to 8) (United States)
Regionalism used in the United States
¿Todavía estás en la escuela secundaria?Are you still in junior high?
el colegio(
koh
-
leh
-
hyoh
)A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).
a. school
El hijo de mi jefe asiste al mismo colegio que mi hija.My boss's son goes to the same school as my daughter.
Mis hijas estudian en un colegio cerca de la casa.My daughters study at a school near our house.
2. (group)
b. association
El Colegio de Arquitectos tendrá su reunión anual en Nueva York.The Architects Association will have its annual meeting in New York.