There are three different use of prefix se-

1.      One function of se- with nouns is to form adjectives that mean “sharing (base), having the same (base)”, where the base is a noun.
Examples:
Dia teman sekelas saya. He is a classmate of mine.
Maya rekan sekamarnya. Maya is his/her roommate.

Alternatively, the se- (base) form may occur as a predicate and be followed by an accompaniment phrase beginning with dengan “with”.
Examples:
Wanda sekelas dengan Yolanda. Wanda is in the same class as Yolanda.
Anaknya seumur dengan anak saya. His/her child is the same age as my child.

Other words in this group include:
searah (dengan) having the same direction/purpose (as)
seimbang (dengan) in balance (with)
sekantor (dengan) sharing an office (with)
selaras (dengan) in harmony (with)
sependapat (dengan) of the same opinion (as)
seranjang (dengan) sharing a bed (with)
setara (dengan) equal (to), matching, on the same level (as)

The combination of prefix se- and a noun sometimes forms an adjective meaning “all, the whole (base)”
Examples:
–          se-Indonesia
all – Indonesia
–          sedunia
(whole) world
– (orang) sekampung
the whole village

2.      Prefix se- means “one”. It occurs with group numbers to form cardinals:
sepuluh ten
sebelas eleven
seratus one hundred

3.      Prefix se- also form fractions, often in combination with per-:
seperempat one- quarter
seperlima one – fifth

Classifier “se-”

A classifier precedes a count noun. If the noun is singular, se- “one” precedes the classifier.
Only three classifiers are in frequent use:

orang used with humans
ekor used with living creatures
buah used with inanimate things

Other examples:
batang used with cylindrical objects such as a pipe, a cigarette
bentuk used with round and curved objects such as a ring, a bangle
bidang used with flat, spread-out things such as a plot of land
biji used with round or elongated objects such as fruit or seed
butir used with small round objects (biji and butir sometimes overlap)
bilah used with sharp things such as knives
carik used with paper
helai used with flat things like leaves, textiles, paper (helai and carik sometimes overlap)
lembar used with flat things (lembar, helai, carik sometimes overlap)
kuntum used with flowers
patah used with words
pucuk used with guns and letters
tangkai used with slender objects such as stems
utas used with rope and thread

ref: Indonsian: A comprehensive grammar; James Sneddon.