That's one of the main differences between English and French: in French, adjectives (small, nice, cheap...) agree in gender and number with the thing or person they refer to. Therefore, they will be either masculine or feminine, and either singular or plural, just like nouns.
Let's see how you pluralise simple adjectives:
Note that regular adjectives take an -s in the plural form.
Case of mixed gender:
In such cases, you will use the masculine plural form of the adjective.
See also:Make most adjectives feminine by adding -e
and the more advanced: Adjectives ending in -s or -x change in the plural forms only when feminine
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Examples and resources
Paul est petit, Sylvia est petite : ils sont petits.Paul is short, Sylvia is short : they are short.
"Plurals in French" by www.frenchspanishonline.com

for an all female class
Ma sœur est intelligente. - Mes copines sont intelligentes aussi.My sister is clever. - My girlfriends are clever too.