It’s a mundane question, maybe even a silly one. Never the less…
Is Sanskrit written left-to-right, or right-to-left?
Left to right and up-down, like English. Scroll down a bit…
Sanskrit is written and read left-to-right and up-to-down. As green_bladder says.
Note that in a regular excercise book (the one with lines that school children use) the words in sanskrit will be aligned to the top of the line. That means, like in English we write abc along the line with the bottom of each letter touching the printed line, in Sanskrit the top of the letters will be touching the printed line, i.e. in Sanskrit, the letters have a horizontal line on top (see links), this line co-incides with the horizontal printed line.
Picture the last (bottom most) printed line of an excercise book. In English, the sentence on the last printed line will be above the last line, and there will be space between the last printed line and the bottom edge of the page. In Sanskrit, if you use the last printed line, you will essentially be writing below the last printed line, hence the bottom of the letters will almost be touching the bottom edge of the page. Therefore, for neatness, you generally don’t use the last printed line when writing in Sanskrit.
An illustration of what Sanskrit looks like: