- Any creature with 1 or fewer HD has their monster hit die replaced by a first character level. (DMG p. 24, MM p. 14). Therefore giving a medium skeleton a level in Warrior would make it a 1st-level Warrior with 1d8 Hit Dice.
- The MM Orc is already a 1st-level Warrior (MM p. 147). Therefore adding another level of Warrior would clearly make one a 2nd-level Warrior with 2d8 Hit Dice.
Giving class levels to skeletons really breaks the rules structure in a number of ways, not only (1) because of their lack of intelligence. There's also: (2) their "advancement" specification which does not include class levels, (3) the fact that their Hit Dice and hit points go down with class levels (as above), and (4) the specification for the animate dead spell which says "statistics for a skeleton depend on its size; they do not depend on what abilities the creature may have had while alive".