A meal provided by the school with 給食

Kanji Explanation: Eating and food with 食

Chiyo is a child prodigy who’s skipped from elementary school to high school.

During her first day of class, Chiyo is invited by a couple of classmates to join them for lunch.

ちよ:: 「小学校(しょうがっこう)は給食(きゅうしょく)だったからお弁当(べんとう)は楽(たの)しいです」; “Since we were provided lunches in elementary school, a boxed lunch is fun.”; Literal: “Because elementary school was provided lunches, a box-lunch is enjoyable.”

Key Points

1給食(きゅうしょく) is a meal provided by a school

Here it refers to the lunches Chiyo ate in elementary school, in contrast with the お弁当(べんとう) she now brings to high school.
The compound joins 給(きゅう), “to supply”, with 食(しょく), “food” or “a meal”.

2小学校(しょうがっこう)は給食(きゅうしょく)だった describes what lunch was like there

The sentence does not literally identify the school as a meal. 小学校(しょうがっこう)は sets elementary school as the topic, and 給食(きゅうしょく)だった says that school lunch was the norm there.
A more explicit version would be 小学校(しょうがっこう)では給食(きゅうしょく)だった.

3〜だったから gives Chiyo’s reason

The past copula だった follows the noun 給食(きゅうしょく), and から means “because”.

給食 (School lunch)

給食(きゅうしょく) means meals regularly provided by a school, workplace, or hospital. It most often refers to Japanese school lunches.

弁当 (Boxed meal)

弁当(べんとう) is a boxed meal packed to eat at school, work, or away from home.