You need to know whom you are addressing. If you are not on a first-name basis, use Usted. If you are on a first-name basis, use tú.
Don Francisco = Usted to him, él about him
Maite y Inés = Ustedes (unless your text book uses "vosotros/vosotras" in speaking to them. You need to know if Maite is a girl = Ellas; or a boy = Ellos
Inés y Alex = Ustedes (or vosotros) to them and Ellos about them
la profesora = Usted to her but Ella about her
un estudiante = tú (if you are also a student and you are on a first-name basis) to him but él about him.
Subject Pronouns are
yo = I
tú = you (singular, familiar)
él = he (or it for a masculine noun such as libro)
ella = she (or it for a feminine noun such as casa)
nosotros = WE (all males or one male and many females
nosotros = WE (all females)
ellos = they (people or things all masculine or one male and many females
ellas = they (people or things, but all female)
Talking ABOUT someone is probably
él = he/it
ella = she/it
ellos = they
ellas = they
In the case of Maite, there are no confusion. Maite is a name of a woman (nobody call a man Maite, no in México, everyone laugh of his name, it´s impossible), you must use "ellas".
In the case of Alex, you can use ellas o ellos, you must know his/her gender.
And, you must use "nosotras" if all are females.
My english is not good enough.
"you must know it gender" I guess