I would guess that you are...
I have been around since even before Cuneiform writing.
The materials needed to create ink have been in the Earth since before Cuneiform writing.
I am usually partially encased in a sheath, of plastic or metal.
Usually the ink will be placed in plastic or metal to be used as a pen.
I support you on your journeys, long and short.
Pens can come in handy for long writing tasks such as essays, but are also quite useful for short tasks, like signing your name.
I come in all different colors, varieties, and sizes.
There are many different colors, varieties, and sizes of ink (when used in pens).
I am pretty useless without my counterparts and nobody wants me alone.
The ink in a pen is useless without something to write on. A pen is usually used with paper, which is a valuable and almost necessary counterpart.
I am also an important part of a common childhood lesson.
Children may choose to use pens to write their notes or do their homework while taking lessons.