Approach the boss at the school and request that he honour the prices that he advertised. If he refuses you should contact the Office of the Director of Consumer Affairs in Dublin. The role of ODCA is to give information and guidance to consumers on their statutory rights.
Their Phone number is 1890 220 229.
You probably will not get your money back but you might persuade the organisation to correct its advertisments or provide their services at the previous price as a gesture of goodwill.
At the end of the day if you still aren't happy, learn your lessons, know your rights, don't go back.
Good Luck