Breakdown of Una decena de estudiantes lee la pancarta ahora.
el estudiante
the student
de
from / of
leer
to read
una
a, an (feminine)
Questions & Answers about Una decena de estudiantes lee la pancarta ahora.
Does una decena mean exactly ten students, or just around ten?
It can mean exactly ten, but it is very often used to mean "about ten." The -ena suffix turns a number into a singular collective noun. Note that because decena is a feminine noun, we use una, regardless of the gender of the estudiantes in the group.
Why is the verb lee in the singular if there are multiple students?
The grammatical core of the subject is the singular noun decena, so strict grammar rules make the verb agree with it: una decena... lee. However, when a collective noun is followed by a plural phrase like de estudiantes, using the plural verb (leen) is also fully accepted and extremely common in spoken Spanish.
Since they are reading it right now, could we use the present progressive instead of lee?
Yes, you could say está leyendo (or están leyendo). But remember that Spanish often uses the simple present tense (lee) for actions happening at this exact moment, whereas English strongly prefers the continuous tense ("are reading").
What is the difference between a pancarta and a cartel?