Lilys

nseika said:
[Source] Strong Gardens (such as Yurigaoka and Odaiba) had education program since kindergarten level.
[Source] [Source] Besides learning about battle and Charm, the focus is in learning cooperation that is needed for effectively perform the Neunwelt tactic's pass work, and it makes a difference between those who joined in later.

This is likely for the newer Neunwelt generation.
The current 3rd year students (at least those who still survives) are called the Duel generation. Their education focus on producing Lily who can go one on one against Huge. The 2nd year and younger are the Neunwelt generation, focusing on battle as team and using the Neunwelt tactics. [Source]. Yuyu is more like the duel generation.
There are also duel revivalist younger generations, such as Shiori, Ichi, Araya, Hiromu, who wants a re-evaluation of the value in reviving the duel generation's education program to train stronger Lily. [Source] [Source]

Ichi and her friends are together since kindergarten.
Araya is, iirc, joined since elementary school grade.
They survive the selection and keep graduating.

There's also example like Tazusa's friend Takahata Masaki (who was the strong candidate for Yuyu's sister) who failed to enter the Lily course when graduating to high school, and she'd rather transfer into other Garden (Alkemira) rather than entering Arsenal (she agrees with Moyu's principle that Arsenal is not dumping ground for those who failed to become Lily).

[Source] Yurigaoka has open selection during the kindergarten, elementary, and high school level. Those who have enough skiller value can be admitted. Fumi attended all the selection just to meet with Lilies and ask for signature.
[Source] There's no public entrance test in middle school, so those who are admitted during that time usually are scouted or by recommendation, so they are considered elites. Mai transferred in from Vietnam, while Soraha got the highest magi capacity among all Lily in the current time.

Kaede took public selection course to enter Yurigaoka High School, regardless of her already excellent track record from her time in St. Mercurius International middle school department.

[Source] Also, in Yurigaoka and Odaiba, Lily in 2nd or 3rd year of middle school can form a temporary legion, called Reserve Squad. They can experience real battles here, before officially be part of the front line after entering high school.