What are the grade boundaries for the practice papers (why did my students do so badly)?

There are no set grade boundaries for the practice papers. 

Grade boundaries are only set for a 'real' exam when it has been taken by the full national cohort of students. The 'awarding' process sets grade boundaries based on actual student performance and the expected outcomes of students based on GCSE performance. This cannot happen with the specimen or practice material. 

This concept of 'comparable outcomes' year to year and between one exam board and another is used so that grades should not be easier or harder to achieve using different assessments.

Please be aware that specimen and practice papers have not been through a live 'standardisation' - a process whereby the actual answers given by students can be considered, before marking, to see if any additions or changes need to be made to the mark schemes. Teachers will probably have found, when marking mocks, some valid student responses which do not appear in the mark scheme. By applying their professional judgement, and crediting these points where appropriate, perhaps in discussion across the department, a form of standardisation in miniature can be achieved. The instructions at the start of the mark scheme give further information that should be used in conjunction with the marks for individual questions. 

A couple of very informative links are included below. The Ofqual one is talking about comparability between boards, but the same ideas apply to comparability between years. The Cambridge Assessment document gives a nice clear overview of the process of marking and grading exams. 

The Ofqual blog - How to choose your exam board 

A simple guide: Assuring Cambridge OCR's Marking Accuracy