The words "sind" & "seid" are just different conjugations of the verb "to be". You would say we areWir sind & they areSie sind, but you would have to say you areIhr seid
The German words "sind" & "seid" both mean 'to be'.
The only difference here is that "sind" is used for the 1st person plural (we) and the 3rd person plural (sie), whereas "seid" is used with the 2nd person plural (you, pl.).
Wir sind in der Schule.
Sie sind in der Schule.
Ihr seid in der Schule.