Independently-Minded Skeleton Tactics

From several editions of DnD, it appears skeletons are typically written as dopes who clackingly follow orders at best, or, at worst, repeat the actions they carried out in life.

Unless their job in life was operating a Rube Goldberg machine of violence, pulling levers to expel burning oils and shoot out pincer claws, this is Not Good Enough. Not by half.

Skeletons need to be smart, dammit, and they need tactics! They, of all the undead, are vulnerable and have relatively low hitpoints, especially compared to the other low-level undead, zombies.

Why Are The Skeletons Smart Though?

1. Wizard with strange politics created them in order to free them; awaits emergence of higher order through giving skeletons more intellect and reasoning capacity.
2. Dully glowing gems chained to their vertebrae grant them decent cognition.
3. Undead brains, burnt and charred to the inside of their skulls in hideous diagrams, afford them a hive consciousness. In groups of 3 or more, the skeletons are quite smart. 'Groups' can be separated by a few hundred feet and retain awareness via one another's senses. This also makes them harder to surprise.
4. Teeth and jaws stained black by a vicious, viscous liquid which allows undead more self-awareness, with occasional bouts of crippling paranoia. The stains become a warning.
5. Partial revenanting gone wrong, giving the skeletons eyes in bleeding sockets, boiled (yet largely functional) brains, scraps of flesh on their bones still. Claw-like hands work as weapons.
6. Devouring the brains - or in their case, mastication alone - has made the skeletons more intelligent. They communicate in morse code by clacking jaws or phalanges against femurs.

Skeleton Small Unit Tactics

Skeletons like to have zombies around as meat shields. They'll chain zombies up and stagger them in rows before them, hiding behind them. This counts as decent cover in whatever way you estimate it, though attacks that miss have at least some chance of hitting a zombie instead. The skeletons fire short bows from behind this mobile cover. The zombies are chained to them with shackles or ropes which allow the skeletons to let one or more zombie loose; they'll do so if their opponents seem to be concentrating fire on them, rather than on the zeds. 

Skeletons will deglove zombies in (frequently successful) attempts to harvest a new skeleton warrior. Degloved tissue will be fashioned into crude armor - likely not improving AC so much as granting the skeleton more hit points, or ameliorating some of their vulnerability to blunt weapons.

Some skeletons' bones aren't connected by anything other than magic or electromagnets. This requires some setup, but in an area with an ancient, whistling pneumatic tube system, skeletons will 'mail' themselves about via tube, arriving in a jumble and self-assembling in 1d4 rounds. In other situations a skeleton will dive behind cover as a loose jumble, providing a small AC bonus. If the bones can land on something soft, falling from a long distance only inflicts half damage.

Skeletons will, of course, throw Greek Fire or ball bearings or caltrops at opponents who are fleshy. Caltrops have no effect on skeletons, other than making their kicks do slightly more damage, should the caltrops get stuck in their metatarsals. These can also aid climbing, which skeletons are adept at (I'd let them succeed on that in a 3+ on a d6 roll). Their toes and fingers are quite suited to teasing out tiny holds during an ascent. 

Skeleton Mass Unit Tactics

Former tinkerers or engineers or sappers probably exist among a large skeleton force. They will make a big show of attacking, leading with zombies and seeking to avoid prolonged engagements, while sappers mine and undermine fortress walls, or while engineers construct trebuchet out of wood and spare zombies. 

They don't get hungry, they don't sleep, they can see you in the dark, they have trebuchet. You better hope in a prolonged siege that their sentience somehow causes some intellectual schism.