Public Health Minister Sudarat Keyuraphan revealed yesterday [Wednesday] that nearly a quarter of the food sold outside Bangkok schools by hawkers is laced with borax.     Drawing on the results of a recent survey into foodstuffs sold outside a school in Bangkok's Huai Khwang district, Keyuraphan warned that borax traces were found in 22% of the grilled pork, fried chicken legs and grilled chicken sampled. A massive 32% were meanwhile found to contain illegal food colourings.