We present the results obtained from our deep survey of lensing clusters aimed at constraining the abundance of star-forming galaxies at z ˜ 6-11. Lensing magnification improves the search efficiency and subsequent spectroscopic studies with the new generation of NIR multi-object spectrographs (e.g. EMIR/GTC). The UV Luminosity Function derived from the photometric sample of candidates does not show the turnover observed at z ˜ 6-7 by Bouwens et al. (2005) towards the bright end. For this reason, the SFR density is consistent with a constant value up to z ˜ 10. This systematic trend with respect to blank fields could be due to field-to-field variance, positive magnification bias from mid-z EROs, and/or residual contamination by spurious sources.