The latest PISA results, due to be released later this morning, focus on maths skills - the area where Welsh pupils particularly struggled in 2009.
PISA tests are designed to assess how students towards the end of compulsory education can apply their knowledge to real-life situations, rather than just a school curriculum.
There are concerns that young people leaving school in Wales are not well equipped to enter the workplace.
Pamela Simpson is Operations Manager for Markes International, a high-tech company which manufactures scientific equipment, in Llantrisant in Rhondda Cynon Taf.
She said: "it is a concern for all employers in this area - the level of numeracy and literacy in terms of school leavers."
"If you are going to fill jobs you need to have that pool of school leavers that are coming through that are educated to that sort of standard. It does make it a little bit harder if you don't have that background level of skills."
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