I was 19 and didn't understand how big of a red flag this was (stupidly) until I became a full time employee. Staff were hired in February and then a second batch in July. The July employees were making $300 more and someone found out and there were quiet complaints in the office. This manager gathered us together (interns and new associates) and told us that because our salary information was outlined in our contract it was considered confidential and we were effectively breaching contract by disclosing our pay and we could lose our jobs.
The February employees didn't get the raise until a year later even though they had 6 months more experience.