Learn Spanish (Spain): Sesenta estudiantes vuelven hoy. - Sixty students are returning today.

No. Unlike uno (which changes to un or una), the tens from thirty to ninety always end in -a and never change for gender. You will say sesenta chicos and sesenta chicas.

I always mix up sesenta (60) and setenta (70). Is there a trick to remember them?

Yes! Sesenta (60) has an 's' in the middle, just like seis (6). Setenta (70) has a 't' in the middle, just like siete (7).

Why is the verb spelled vuelven if the dictionary says the verb for "to return" is volver?

Volver is a "stem-changing" verb. In the present tense, the -o- in the middle changes to -ue- for most forms, including the "they" form used here for the students.

How do we know if estudiantes refers to male or female students here?

Just looking at this sentence, we don't! The noun estudiante is used for both men and women. If we added a word like los or las (the), that would tell us the gender, but the number sesenta doesn't give it away.