Children are subject to compulsory schooling.

A shared obligation with state-specific rules.

Attending school gives children knowledge and access to educational pathways. The obligation concerns regular attendance under the applicable rules; it is not merely an optional recommendation to parents.

According to the education ministers' conference, full-time compulsory schooling usually lasts nine or ten school years. A vocational-school obligation may follow. Compulsory schooling is therefore broader than the idea that every obligation ends after one fixed nationwide number of years.

The other three answers

The other terms refer to different matters.

Compulsory schooling does not mean state schools only.

Private schools can also fulfil the obligation under the applicable law. Article 7 protects their establishment subject to particular rules; an arbitrary private learning activity is therefore not automatically an approved substitute school.

The question does not require an assessment of individual exemptions, deferred entry or state-law details. The responsible school and current state rules are decisive for a particular transfer or starting date.