From Halopedia, the Halo wiki

Originally posted at Wikianswers, but it became unreadable. Bold type used for skimmability.

  • In October 2001, Fall of Reach introduced the original 75 Spartan-IIs, conscripted in 2517 from a pool of 150 candidates who were assigned the tags Spartan-001 through Spartan-150. Mendez leaves to train a new class in 2525, but no mention of this is made after the time skip to 2552; the number of Spartan killed during the gap would have to exactly equal the number trained (minus five?) for it to work out. Thus it was unclear whether Class Two was in or out.
  • In November 2001, Halo 1 was released. In all material directly surrounding the game (and later Halo 2), a big deal was made of John-117's status as the last or essentially the last Spartan, with no indication that more Spartans after him could be in training. Class Two was out.
  • In 2004, I Love Bees introduced Yasmine Zaman, conscripted in 2537. Class Two was in.
  • ILB was declared non-canon partway through its run. Class Two was out.
  • In 2005, Dead or Alive 4 featured a Spartan named Nicole-458; she was declared to be non-canon from the beginning, but elements of her backstory (the Spartan program itself and the Earth Defense Grid) were clearly canon, suggesting that Class Two might be as well.
  • In July 2006, references to ILB showed up in the Halo Graphic Novel, and Frank O'Connor told an interviewer that ILB was going to be recanonized. Class Two was in.
  • In October 2006, Ghosts of Onyx talked about how Class Two had been "postponed indefinitely", replaced by the allegedly more efficient Spartan-III program. Fans speculated that Yasmine Zaman was now canonically a Spartan-III. Class Two was out.
  • In June 2009, the first Halo: Reach trailer mentioned the callsigns Sierra-259 and Sierra-320, consistent with the established callsigns for Spartan-IIs and inconsistent with those for Spartan-IIIs. Some fans argued that there was no way to be sure they were even Spartans, but otherwise Class Two appeared to be in.
  • In October 2009, the Halo Encyclopedia was released, featuring detailed coverage of the Spartan-II and III programs with no mention of a second class. Yasmine was listed as part of Class One despite this being chronologically impossible. Class Two was out.
  • In December 2009, the second Halo: Reach Trailer revealed that Sierra-259 and Sierra-320 were Spartans, wearing the Mjolnir armor characteristic of Spartan-IIs. Although blaming the writers might not be fair for this one, Class Two was in.
  • Later in December or in January 2010 (not sure), further material was released that presented them as Spartan-IIIs, with the armor being thanks to subterfuge. Class Two was out.
  • In February 2010, the Halo Legends short Homecoming introduced a Class One S-II named Ralph-303, implying a massive retcon whereby rather than there being two classes, Class One was now much larger than originally presented.