Your working is correct. 
For GCSE and A/AS level, they uses different periodic table.
Your periodic table is for A/AS. 
For GCSE
0.25% of 100 tonne = 250 kg
mol of sulphur present = 250 000 / 32 = 7812.5 mol
Reaction ratio Mg : S = 1: 1
therefore mass of of Mg needed = 7812.5 mol x 24 =
187.5 kg
For A / AS Level
0.25% of 100 tonne = 250 kg
mol of sulphur present = 250 000/ 32.1 = 7788.2 mol
Reaction ratio Mg : S = 1: 1
therefore mass of Mg needed = 7788.2 mol x 24.3 = 189.25 kg
Best regards,
Chin Seng