Students are killers! After the painful death op three raspis I decided to design a short circuit resistant buffer board. A little googling pointed to this page from brian dorey. The mentioned TXS0108E sounded interesting and the design was straightforward. Because of the raspis multiple GND Pins it was possible to come down to only 20 pins and so freeing some space on the breadboard.
I tested the board with 1-wire, I2C and EIA232 and it worked very well with 3.3V. With 5V it is a little noisy. The TSX0108E is designed for high-impedance loads! It has has an internal serial resistor of appr. 4k and can't deliver high currents. If the load-impedance is too low, the output signal is oscillating!! More info in this TXS application note!
The PCB was designed with kicad.