The Google Highly Open Participation Group was a precursor to GSoC. It concluded in 2008. However, it is more like the Google Code-in, because the focus was on pre-university students.
Here is a letter from Timothy Clemans:
"Dear Python's Project for the Google Highly Open Participation group,
I am a 17 year old home schooled high school student in Seattle, Washington and contributor to a Python based open source mathematics software project called Sage (http://sagemath.org). Onrej Certik, creator of SymPy (http://sympy.org), suggested that the Sage project participate in this contest as a Python project. I think that is a wonderful idea because in the Sage project there are many interesting and accessible problems to work on and numerous ways for pre-university students to contribute time. I would like to get Sage involved in the contest by introducing Sage, planning Sage involvement, and writing and supervising tickets. I do not want to be a contestant.
Sage is a massive extension to Python for all kinds of mathematical computing. The goal of the Sage project is to develop an open source alternative to the math software Maple, Mathematica, MATLAB, and Magma. To achieve this goal in a reasonable amount of time, the Sage developers did not reinvent the wheel. Instead, the Sage project has produced a system with a wide range of functionality in less than three years by maintaining a distribution of the best available math software and writing a Python library that unifies the software and libraries and eliminates much of the complexity of many of these packages for the end users.
Many people first started seriously using Python because of Sage. At every introductory talk on Sage, the advantages to using Python over other languages and its popularity are well emphasized. Thanks in great part to Python, Sage is an excellent system for teaching students about both math and computer science.
Sage has a web environment called the Sage Notebook, and two free public notebook servers are available at http://sagenb.com and http://sagenb.org. A Sage worksheet is similar to a Mathematica notebook, and although it lacks many of the neat features of Crunchy, it does provide a fairly robust notebook system. Some of Sage's features include user accounts, worksheet sharing, tab completion, infinite loop survival, two and three dimensional graphics, interactive documentation, and public notebook security.
The Sage Notebook is also an excellent platform for creating mathematical art. A good example of that is a worksheet called “New Found Spin” at https://sage.math.washington.edu:8102/home/pub/11/.
Although most of the focus of Sage development is on aiding cutting edge mathematical research, Sage has good support for elementary algebra, geometry, trigonometry, and calculus. Also, some of Sage's upper level mathematics functionality is accessible to many high school and college students. For example, in the summer of 2006, under the direction of William Stein (lead developer of Sage) 24 talented high school students used Sage via the notebook in a computer lab to explore the congruent number problem and in the process were introduced to the Birch and Swinnerton-Dyer conjecture (http://wstein.org/simuw06/). Then, in the next summer, another 24 students used Sage to comprehend Riemann's Hypothesis (http://wstein.org/simuw/).
I think that there are many ways that pre-university students can contribute to Sage.