Every civil servant pays for parking. That teachers did not pay was an anomaly. Whether the anomaly ought to have been rectified is another matter.
I personally think it would have been wiser to let things be and justify it as an exception to the rule. There are several arguments that could have been used. But the civil service, particularly in its upper reaches, has a visceral aversion to anomalies of any kind.
Besides to try to justify an anomaly would require a fight with the PSD and Auditor-General. But it is not the requirement to pay that needs justification.