My workplace, an ITeS in Ghaziabad, works 6 days a week and has 9 hours working including 0.5 hours of lunch break.

This comes out to be 51 hours a week with 3 hours of lunch break.
Most of the employees are working under consultation agreements and are given professional fees and not salaries.
Are these working hours legal under the applicable laws?
If not, how can this be conveyed to the management/ HR?

Your establishment is under the UP Shop & Establishment Act and as per Section 6, more than eight hours of work per day constitutes overtime and has to be compensated at double the wages. Being a private ITES set up, it may not be advisable to take up cudgels in the matter but this blatant violation of law could be brought out subtly in the informal group talks and sooner or later it will reach HR & Management, so you can expect some reaction. The Labour Inspector can initiate action in this matter once he knows of this violation of law.