In this episode of The Legion Clubhouse, we take a look at Adventure Comics #305 and #306, and we meet the kids that didn't get into the Legion of Super-Heroes.

In the city of Metropolis, in the 30th Century, there exists one of the most amazing clubs of all time! It's members are teen-aged youths, each possessing on special super-powers! The club members have vowed to use their fantastic power to battle crime... This rocket shaped building is the Legion Clubhouse!

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Adventure Comics #305
Secret of the Mystery Legionnaire!
February 1963
w: Jerry Siegel
a: John Forte
With the recent demise of Lightning Lad, the Legion of Super-Heroes begins looking for an adequate replacement to fill the void left by the heroes death. After testing a series of would-be Legionnaires who's powers either turned out to be duds or artificial, the Legion is met by a costumed youngster calling himself Marvel Lad.

Wanting nothing but the best possible candidate to take the late Lightning Lad's place, the Legion puts Marvel Lad on a series of complex tests to see if he stands up to the Legion's values and codes. When Marvel Lad is able to complete all the missions the Legion asks him to reveal the origins of his powers before he is introduced into the group.

The next day Marvel Lad arrives and reveals that he is really Mon-El in disguise. He then reveals to an astonished Legion that Brainiac 5 had developed a new version of serum XY-4 to try and cure Mon-El of his lead poisoning that forced him to reside in the Phantom Zone. To test out if the serum will work, but not get the other Legion's hopes up, Mon-El took on the guise of Marvel Lad and try out for Legion membership, that way if the serum didn't work the Legion wouldn't be disappointed.

Having passed the Legion's tests, Mon-El is welcomed into the Legion as their new member, and Mon-El returns to the Phantom Zone one last time to get the last laugh on the criminals trapped there who have taunted and tormented him during his time living within the Zone.
via DC Wikia

Adventure Comics #306
The Legion of Substitute Heroes
March 1963
w: Edmond Hamilton
a: John Forte
After being rejected for membership in the Legion of Super-Heroes, five super-powered teens decide to join together to become the Legion of Substitute Heroes. They are: Night Girl, who has super-human strength but only during the night; Fire-Lad who can breath fire; Chlorophyll Kid who can control plant life; Polar Boy who has freeze powers; and Stone Boy, who can turn into an immobile, and indestructible stone statue at will.

Building a secret base, the Substitute Heroes decide to try and be on call to help the Legion of Super-Heroes when there is trouble. However, whenever there is a mission to be accomplished, the Legion seems adept at solving the problem on their own. However, when the Legion is busy fighting robots in space, the Earth is attacked by a number of Plant Men. Easily defeating them, the Legion of Substitute Heroes tracks the race to their planet, where they find that the Plant Men are preparing for a full scale invasion of the Earth. The Substitute Heroes work together and crush the Plant Men's scheme, using their own reproduction methods against them. Returning to the Earth as unsung heroes in light of the Legion's recent victory over the robots, the Legion of Substitute Heroes decides to remain together, but deciding to keep their victory secret so as not to steal any of the Legion's glory in the hopes that some day they can act publicly as the Legion of Substitute Heroes.
via DC Wikia