noun
senior class (kindergarten, preschool)
Refers to the class for the oldest children in a kindergarten or preschool, typically 5- to 6-year-olds.
うちの子は来年年長組になります。
My child will be in the senior class next year.
年長組の子どもたちは運動会でリレーをしました。
The children in the senior class ran a relay at the sports day.
The middle class (usually 4- to 5-year-olds), between 年少組 and 年長組.
Compound of 年長 (nenchō, 'older, senior') and 組 (kumi, 'class, group').