Pascal is a structured language specially designed by Niklaus Wirth for learning programming and computer science.
Why Pascal? 
Pascal is accessible to beginners, its use of English-like syntax and avoidance of curly brackets and other arcane symbols being similar to Python and VB. Modern versions such as Delphi and Lazarus are very flexible and powerful, enabling creation of console programs for AS as well as very easy and rapid development of Graphical User Interfaces for A2 projects. They also provide full support for Object-Oriented Programming as required for A2. Pascal is the only language on the AQA list which enables practical experience of pointer programming.
The open-source Free Pascal/Lazarus project provides a free, high quality compiler and development environments for Windows, Linux and Mac OSX.
The AQA textbook and exam papers use Pascal or Pascal-like pseudocode to describe alogorithms.
Why not Pascal? 
The use of Pascal as a teaching language in universities peaked in the 1980's and is now probably extinct in the UK and US.
Pascal was never intended as a language for commercial development though Delphi is used by some software companies especially in Eastern Europe.
Delphi or Free Pascal? 
Lazarus/Free Pascal is open-source, multi-platform and free!
Delphi runs only on Windows and costs money (£23 per concurrent licence for educational use). However the Development Environment is more polished, the compiler is much faster and database connectivity is easier and more powerful.
Getting Started 
Download Lazarus from http://sourceforge.net/projects/lazarus/files/
Pascal fits the AQA specification very closely:
- it's very easy to create console programs using a purely procedural approach (no objects or classes needed)
- the use of value and reference parameters matches that described in the specification
- it supports static arrays, records and sets as native data types
- procedures and functions are distinguished and named as in the specification
- traditional text files and typed files are supported
Pascal enforces strong typing and explicit variable declaration which encourages students to think about data types before they code
Modern Pascal compilers provide full support for Object-Oriented programming with a structure and syntax similar to that used in textbook and exam questions.
The ability to use pointers explicitly, although indicative of Pascal's 1970's origins, does make it easy to teach some of the more difficult data structure topics: linked lists, trees, use of the heap.
Delphi and Lazarus excel at the rapid development of graphical user interfaces, allowing the student to spend much more time on the underlying algorithms in their project. Only Microsoft Visual Studio competes with this.
Online resources 
- Delphi Basics a very clear and accesible reference, suitable for beginners
- PP4S Pascal Programming for Schools, includes introductions to Lazarus and Delphi Object Pascal environments
- Pascal Programming at Wikibooks
- Delphi For Fun full of excellent example programs, including complex algorithms relevant to A2
- About Delphi includes a very good course on connecting to databases
Development environments 
- Embarcadero Delphi commercial and proprietary Object Pascal compiler and IDE
- Lazarus/Free Pascal open source Object Pascal compiler and Lazarus IDE
Pascal's long history in universities and industry mean that it is thoroughly documented. Useful printed books are as follows:
|Learning to Program in Pascal and Delphi||1904467296||specially written for A Level students|
|Delphi in a Nutshell||1565926595||reference guide for more experienced programmers|
|Discover Pascal in Delphi||0201709198||beginners|